The technique applied for the first time in the Melissa virus was developed subsequently by viruses like VBS / Freelink who, on the contrary to its predecessor, sent itself to all contacts recorded in the address book of an infected PC. This was the beginning of a new generation of worms, that were able to send themselves to all the contacts found in the address book in Outlook of the infected computer. From all those worms, the one that really stands out is the VBS / LoveLetter, widely known as I love You Virus, which first appeared in May 2000 and caused an epidemic with losses estimated at 10,000 million euros. To attract the attention of users and to contribute to its spread, this worm sent itself via an e-mail message with ILOVEYOU title and an attachment named LOVE-LETTER-FOR-YOU.TXT.VBS. When the user opens the attachment, the computer was infected.

Besides Melissa, in 1999 another type of virus appeared, which was also a milestone in the history of viruses. In November of that year the VBS / BubbleBoy virus (written in VB Script) was created: a new type of worm which spread over the Internet, without the user clicking on an attachment. To automatically run itself when the user opened or displayed a message, the virus took advantage of a security problem in Internet Explorer 5. Successor of this worm in 2000 was the JS / Kak.Worm, which spread by exploiting the automatic signature of Microsoft Outlook Express, which enabled it to infect computers without requiring the execution of a file from the user. These were the first samples of a series of worms that were subsequently enriched with new members – worms that could attack computers while users were browsing the Internet.

While Windows were evolving as an application designed to facilitate the management of the DOS in a 32-bit operating system, the virus developers returned to using assembly as the main programming language to create viruses. Versions 5 and 6 of Visual Basic (VB) were, together with the Borland Delphi (Pascal language environments for Windows), the preferred development tool for creators of worms and Trojan horses. Then Visual C came into play, which offered a powerful application programming language for Windows. Quickly C was adopted by the creators of viruses, Trojan horses and worms. Viruses based on the Visual C language acquired unprecedented power, supplanting all other types of viruses. Although the characteristics of worms have changed with time, all have the same goal: to spread to as many computers in the shortest possible time.

Over time, Visual Basic became extremely popular and Microsoft implemented it as part of the functionality of a separate tool: an “interpreter” capable of executing script files that contained code with similar syntax.
Simultaneously, with the establishment of the 32-bit Windows platform, the first script file viruses were born: These were hostile software hidden inside a plain text file. Script file viruses showed that the executable files (files with extensions. EXE and. COM) were not the only ones that could carry viruses. As we have already seen with BAT files for viruses, there are other means of spreading a virus, fully justifying the assumption that everything that can be executed either directly or through an interpreter, may contain a hostile software. Specifically, the first viruses that could infect the macros contained in the applications of Microsoft Office came into the scene. As a result, Word, Excel, Access and PowerPoint have become vehicles for the spread of lethal weapons, which destroy data even when users simply open a document.

• File Infector Virus – Infects program (object) files. System Infectors that infect operating system program files (such as COMMAND.COM in DOS) are also file infectors. These type of viruses can attach to the front of the object file (prependers), attach to the back of the file and create a jump at the front of the file towards the virus code (appenders), or overwrite the file or portions of it (overwriters). A classic example is Jerusalem virus. A bug in early versions caused it to add itself over and over again to files, making the increase in file length detectable.
• System Infector Virus – A somewhat vague term. Some use the term to indicate viruses that infect operating system files, or boot sectors, such that the virus is called at boot time, and has or may have pre-emptive control over some functions of the OS. In other usage, a system infector modifies other system structures, such as the linking pointers in directory tables or the Windows Registry, in order to be called first when programs are run. Many email viruses target the Registry, such as the Magistr virus.
• Boot Sector Infector – Infects the Master Boot Record, System Boot Record, or other boot blocks on physical disks. Boot Sector Infector viruses usually copy the existing boot sector to another unused sector, and then copy themselves into the physical first sector, ending with a call to the original programming. Examples are Brain, Stoned, and Michelangelo.
• Email Virus – A virus that specifically, rather than accidentally, uses the email system to propagate. While virus infected files may be accidentally sent as email attachments, email viruses are aware of email system functions. They generally target a specific type of email system (e.g Microsoft Outlook), harvest email addresses from various sources, and may append a copy of themselves to all outgoing email, or generate email messages containing copies of themselves as attachments. Some email viruses may monitor all network activity, and followup legitimate messages with messages that they generate. Some email virus examples are Melissa, Loveletter, Hybris, Christmas etc.
• Multipartite – Originally this term was used to indicate a virus that was able to infect both boot sectors and program files. Current usage tends to mean a virus that can infect more than one type of object, or to reproduce in more than one ways. Examples are Telefonica, One Half, and Junkie.
• Macro Virus – A virus that uses macro programming of an application such as a word processor (Most known macro viruses use Visual Basic for applications in Microsoft Word: Some are able to cross between applications and function in, for example, a Powerpoint presentation and a Word Document). Macro viruses can operate across hardware or operating systems as long as the required application platform is present. Examples are Concept and CAP. Melissa is also a Macro virus in addition to being an email virus.
• Script Virus – Script viruses are generally differentiated from macro viruses in that script viruses are usually standalone files that can be executed by an interpreter, such as Microsoft Script Host (.vbs files). A script virus file can be seen as data file in that it is generally a simple text file, but it usually does not contain other data, and generally has some indicator (such as the .vbs extension) that it is executable. Loveletter is an example of script virus.

Source: CISSP CBK Review Seminar Book

But whats actually is a Trojan Horse in our modern computer era? A Trojan Horse is a destructive program that masquerades as a harmless application. Unlike viruses, Trojan Horses do not replicate themselves, but they can be just as destructive. One of the most dangerous examples of a Trojan is a program that promises to rid your computer of viruses but instead introduces viruses into your computer.

When a Trojan is activated, numerous things can happen. Some Trojans are more annoying than malicious. Some of the less annoying Trojans may choose to change your desktop settings or add silly desktop icons. The more serious Trojans can erase or overwrite data on your computer, corrupt files, spread other malware such as viruses, spy on the user of a computer and secretly report data like browsing habits to other people, log keystrokes to steal information such as passwords and credit card numbers, phish for bank account details (which can be used for criminal activities), and even install a backdoor into your computer system so that they can come and go as they please.

The Trojan can be tricky. Who hasn’t been online and had an advertisement pop up claiming to be able to rid your computer of some nasty virus? Or, even more frightening, you receive an email that claims to be alerting you to a new virus that can threaten your computer. The sender promises to quickly eradicate, or protect, your computer from viruses if you simply download their “free”, attached software into your computer. You may be skeptical but the software looks legitimate and the company sounds reputable. You proceed to take them up on their offer and download the software. In doing so, you have just potentially exposed yourself to a massive headache and your computer to a laundry list of ailments.

To fight Trojans as much as possible, follow these guidelines:

• Install reliable anti-virus software. It is also important that you download any updates frequently to catch all new Trojan Horses, viruses, and worms. Be sure that the anti-virus program that you choose can also scan e-mails and files downloaded through the internet.
• Remain diligent: Trojans can infect your computer through rogue websites, instant messaging, and emails with attachments. Do not download anything into your computer unless you are 100 percent sure of its sender or source.
• Consider installing a firewall. A firewall is a system that prevents unauthorized use and access to your computer. A firewall is not going to eliminate your computer virus problems, but when used in conjunction with regular operating system updates and reliable anti-virus software, it can provide additional security and protection for your computer. Also, if the firewall is configured to block outbound communication from your computer towards the Internet, you can stop any malicious traffic from a Trojan software trying to steal personal info and send it out to the Internet.
• Ensure that your operating system is always up-to-date. If you are running a Microsoft Windows operating system, this is essential. Windows VISTA is the best OS you can install nowadays since it takes security very seriously and provides an arsenal of tools to detect and fight Trojans and other malware.

Nothing can guarantee the security of your computer 100 percent. However, you can continue to improve your computer’s security and decrease the possibility of infection by consistently following these guidelines.

• Virus: A virus is a malware program that is loaded on your computer without your knowledge, with the intent of doing some damage to your system. It normally attaches itself to another program or data file in order to spread and reproduce itself in other areas of the computer without the knowledge of the user. Normally a virus enters your computer through a spam email which has attachments (pictures or files) or by downloading infected programs from malicious sites. A virus can damage files or cause your computer to behave strangely.
• Warm: Warms are memory-resident malware threats that can spread across networks by exploiting possible Vulnerabilities in the TCP/IP stack implementation of the OS and/or specific applications. They load themselves into the memory of a remote system and then execute themselves … all without ever being written to a disk. A warm therefore can live on its own and propagate by copying itself from one computer to another. Worms can harm a network, can consume tremendous bandwidth, and can shut a computer down.
• The difference between viruses and worms is that a virus cannot replicate itself like a worm, and it usually affects the computer it has invaded. A worm acts autonomously, and uses a computer network in order to multiply itself and to send copies of itself to other systems. A virus needs a user action (e.g download of infected file, run a program etc) in order to propagate and spread itself.
• Adware: An adware is certainly less threatening than a Warm or Virus. Typically, adware components install alongside a shareware or freeware application and bring targeted advertisements to your computer. These advertisements create revenue for the software developer. Adware displays web-based advertisements through pop-up windows or through annoying advertising banners. It can slow down your computer by consuming cpu, memory and bandwidth.
• Spyware: A spyware is closely related with adware, since it is usually transmitted also via freeware/shareware applications that we often download. As soon as it is downloaded it installs itself in your PC without your knowledge, and starts to monitor your internet activity. The monitored information is then transmitted to a third party, in most cases to companies which are interested in creating your personal profile. Later on, it will start sending you advertising or other data.
• Trojan: It is a program that appears to be working properly but actually causes damage in some form. It is called a Trojan because it can enter your computer through operations considered harmless, for example, through a game or even through a virus tracking program, and so it can trick the user by hiding the underlying activity. Like the ancient Trojan Horse, the Computer Trojan can secretly open ports on your Home Network. In general, Trojan horses cannot multiply themselves as warms do, but they rely on people to pass them around. The idea is to make the program look like it’s something harmless, like a screen saver or joke, so it gets sent around.

Bottom line is that you don’t want any of those little nasty codes on your computer. Fortunately there are several tools in the security “Arsenal” that can help you fight all these malware threats:

Antivirus Software

Antivirus software is the countermeasure program used to “inoculate” computer viruses. The software is installed on your PC and checks against viruses all files on your computer as well as files attached in incoming emails. If it finds viruses, it immediately informs you and, in most cases, it quarantines/cleans the infected files. The Antivirus can detect and clean both viruses and worms.

The Antivirus tool is a “must-have” software if you want to browse the Internet safely. The Three Best Antivirus Software available on the market today (which I am currently using on my personal computers) are the following:

1. Norton from Symantec
2. Kaspersky Anti-Virus Products
3. Trend Micro

Complete Security Suite

A complete security suite is a collection of tools for defending against malicious threats for your computer. The security suite usually includes antivirus, antispam, firewall, antispyware etc. An example of such a suite is The Norton 360 All-In-One Security

Although a security suite can help you prevent more types of malware compared with the standalone antivirus, the disadvantage of this software is that it takes more computer resources compared with a single antivirus solution.