A computer virus is a piece of code inserted into programs to cause some unexpected and usually undesirable event.
Viruses vary in the harm caused to the infected computer.
Some may simply display a message on your screen while other affect data, erase files or cause your hard disk to require reformatting. These viruses infect code found in certain system areas on a disk. They attach to the DOS boot sector on diskettes or the Master Boot Record on hard disks. They are triggered when you boot from the infected disk. These viruses attach themselves to program files.