All About Viruses
 

A computer virus is a program designed to spread copies of itself onto multiple computers by infecting files or system areas of hard drives and floppy disks. Nasty viruses also can contain “payloads� in the form of malicious code that may alter, corrupt or delete files on a hard drive and make them inaccessible to you. Viruses typically try to operate invisibly, unbeknown to you, until it’s too late and the damage is done.
And viruses aren’t the only source of “colds� your PC can catch. You should watch out for two other infectious beasts: Worms (code that exists simply to replicate itself, not to damage files) and Trojan Horses (secret programs that allow access to a system through a remote connection). Worms are the least offensive of terrible trio. Trojans, meanwhile, are in a different class. They’re often spread the same way as viruses and worms (through email attachments or downloaded executables), but they require human intervention to be truly destructive.