Here's a nice explanation for you: http://www.webopedia.com/DidYouKnow/Internet/2004/virus.asp Viruses don't tend to come in types, more in different forms with different capabilties. That page should supply most of the information you need. There isn't a certain "type" of harmfull virus, it all depends on what it was programmed to do. Here's a small list of capabilities: 1. Broswer and desktop hijacking - Sets your homepage to an unwanted site, usually selling rogue software, or porn. Or sets your desktop background to an image, usually saying something like "Windows Has Detected A Serious Error, Please Download Anti-Virus Software From Www.blablabla.com" also a rogue software website selling you more viruses than you started with, they will normally offer a free "scan" which will always return results, then when you click to remove, you have to pay. NEVER PAY IN HOPE! Check the website & software name out on google and forums for people who have had similar or the same problems. Most will offer advice on how to remove and fix it. "Hijack This!" is free and reliable. 2. Popups - Just bombards your computer with pop ups galore selling everything from diet pills to penis enlargers (these will often be sent into your e-mail inbox too) 3. Slowed performance - They can run programs in the background which you don't know about causing your computer to seriously slow down and probaly eventually crash. 4. Backdoors - allowing the creator to invade your system having the free roam of all your files, webcam, e-mail .etc 5. Keyloggers - Records all keystrokes from your keyboard and sends them via the internet to the creator/s. This can steal all your credit card details, passwords, confidential letters...etc, you catch my drift. Anything you type basically! 6. Destruction - Can just delete or infect important system files, causing all kinds of problems and possible data corruption. Also it is possible to turn off the computer fan resulting in overheating, almost certainly destroying your processor and possible fire! Put all of these together in a nice little package and you've got yourself a recipe for disaster. Hope this info helped a little bit, if your wanting to learn in depth about different viruses and there history .etc then google has alot to offer. Hope this helped, Bambi

Computer viruses are generally defined as a program inputted into a computer that allows replication of the program installed. As it replicates, the program intentionally infects the computer, typically without even the user knowing about the damage being done. A virus, unlike worms or Trojan horses, needs an aid to transfer them to computers. Viruses usually take a large amount of computer memory, resulting into system crashes. Viruses are categorized to several parts based on its features. Macro Viruses Network Viruses Logic Bombs Companion Viruses Boot Sector Viruses Multipartite Viruses