Not all computer viruses are equal. After all, virus writers are often trying to impress their colleagues. Nobody would be impressed if they all did the same thing. So, yes, there is a lot of variety, not only in what the viruses do, how they spread, but also in methodology.

We can classify this whole area into the following:

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• Computer virus.
  A small piece of software that is designed to piggyback on other programs to work and spread. For example, a virus may be designed to attach itself to a particular operating system file. Whenever a particular operation is performed on that computer, the file is run. The virus is also run, leading to the virus spreading and doing what it was designed to do.
• Email virus.
  An email virus is designed to spread specifically through email. It does not tack itself onto other software. Instead it takes advantage of email. For example, it may be spread by email attachment. Whenever a user opens the attachment, it will run the virus. Typically the virus will automatically and invisibly email itself to other email addresses in your email software, such as your contacts. Some email viruses are even designed to not specifically require the user to open the file. Simply by previewing the email in the email viewer, it could execute the virus.
• Trojan Horses.
  A trojan is a program that disguises itself as one thing while actually doing another. For example, a trojan may masquerade as a movie file, screensaver, or perhaps a picture file. When “opened”, the trojan will infect the PC.
• Worm.
  A worm is a piece of software that is designed to take advantage of security holes in software or networks in order to spread. When it spreads to a computer, it will begin to scan the network for other computers with the same security hole. If it finds an available host, it will spread itself.

Within these virus types, we have some other different classifications:

1. Boot Virus: A boot virus is designed to infect the master boot record of the hard disk. The master boot record is that area of the hard drive which is responsible for booting your computer when you turn it on. When a virus is able to infect this section of your hard disk, it will be executed immediately after you turn on your computer, before you operating system even has a chance to load.
2. Program Virus: This is a virus which is run just like any other program on your computer. Such a virus may have a file extension like BIN, COM, EXE, DRV (driver file) or SYS (system driver). When the program is run, the virus is executed as well and becomes active in system memory.
3. Stealth Virus: This is a computer virus which is designed specifically to avoid detection by anti-virus software. It may do this by redirecting the drive’s read/write head to another sector if the sector the virus is on is accessed.
4. Multipartite Virus: This is a combination of boot virus and program virus. The virus is executed as a normal program but, when run, will then infect the master boot record. A program which is designed to install a virus is often called a dropper.
5. Polymorphic Virus: A virus that will encrypt itself in different ways so as to look different each time it is spread. This makes detection harder for anti-virus programs.
6. Macro virus: This is a virus designed to take advantage of the macro capability of many types of documents, such as Microsoft Word. When the document is opened that is infected, the macro virus is activated and run. The virus may be designed to infect the Normal.dot file (the document template for all Word documents created by default) which would mean that all Word documents you create after infection will also contain the macro virus.
7. Active X: ActiveX is a Microsoft-created way of allowing small applets to run on your computer directly from the web browser. For example, when you run Windows Update inside of Internet Explorer, you will notice that it is updating your computer by way of Internet Explorer. Well, this same interface opens up a whole world of potential security breaches. ActiveX could be used to run other software code as well, even if not created for helpful purposes.
8. Cross-Site Scripting: This is a type of vulnerability that is more or less exclusive to web-based applications. Essentially, a security hole would exist in the website which would allow the attacker to inject their own programming code into the webpages viewed by other people.

As you can see, there are a lot of very creative ways to breach the security of your computer. As software on our computers get larger and introduce more features, more and more effort is made to make things more convenient. However, convenience sometimes means opening things up to security breach. Since virus authors are just as creative as the programmers of your favorite software, there are about as many avenues of attach as there are programs available to your computer.

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