Protect Your Computer from Viruses and Trojan Horses
Viruses are nasty little pieces of computer programming code that
make your computer do things you didn't want and didn't authorize. The
category of unwanted critters is called malware, or malicious software.
Malware comes in different varieties, including true viruses and
programs called Trojan horses.
True viruses are self-replicating: They attach themselves to a file,
and every time the file is opened, the virus is opened and activated.
Viruses also can reside in computer memory, and each time your computer
opens or modifies a program, it activates the virus. Over time, left
unchecked, the virus will replicate itself onto many programs.
A Trojan horse technically isn't a virus because it doesn't
replicate itself and spread to other programs. Instead, it's a program
that appears to be one thing, but is actually something else. For
example, you could download a program that you believe to be a new
game. Once launched, however, the program is really a "stealer" — a
program designed to steal your user name and passwords.