A computer virus is a program designed to replicate and spread on its own, preferably without you knowing it exists. Some have been designed to do much damage to your data, while others, have been designed to look like hardware problems and can cause physical damage if undetected and allowed to run when activated. The first viruses were probably written as games. Some came into existence in the test labs and accidentally found their way out into the world. And unfortunately, some were written with the intent to damage and even destroy computer data.
Viruses can spread in several ways.
Some duplicate themselves when an infected file is opened. Others infect the boot sector on your computer's disk drive and then infect other disks as they are accessed. A virus that has infected the boot sector of a disk can travel on a data disk as well as a program disk. You can make the whole scan operation as transparent and painless as possible by installing a scanner, such as McAfee, which, automatically loads on power-up, continuously scans without the user being aware that it is running in the background and is always on guard looking for viruses. This is the route most people choose.