Cert was number 1 on my list. A list of viruses can be found here. A computer virus is usually a short program that was designed by an individual to disperse itself and make copies of it in other computers. Its purpose is usually to disrupt a computers normal operations. A virus is usually spread via floppy disks, networks and online services and is usually in the file or boot part of a disk.
Some viruses are harmless but many are troublesome.
Historically it has been known or suspected that individuals called Hackers are responsible for originating computer viruses. These Hackers are usually smart individuals who want to access other computers either with the purpose of curiosity or with the intent of acquiring or using information. Computer viruses work in your computer or on you disk by setting themselves up so when you try to access a program you are sent to the virus. You cannot obtain you information because the virus has changed the accessing information on your disk or computer. Some viruses work by bombing a server or site with unwanted mail.