• An infected computer.Janis Christie/Photodisc/Getty Images
  
  It's hard to imagine life without computers. They automate our industries, run global communication networks, deliver essential community services and allow us to get more done in less time. That's why viruses are such a threat. These malicious self-replicating programs are used to overload networks and systems with junk data, to corrupt or delete resident files and programs, to steal personal information and even to take down hardware. The viruses in this last category are the most malicious of all.
  
  

Michelangelo

• Michelangelo made a big splash in January 1992 when a computer supplier shipped 500 PCs that were infected with the virus. A public panic followed soon after when antivirus expert John McAfee estimated that Michelangelo could damage from 50,000 to five million personal computers. Michelangelo never did live up to the hype, because its malicious behavior is also its Achilles' heel. As the worst MS-DOS virus ever, Michelangelo attacks the boot sector of a victim's C drive and replicates itself by infecting floppy disks. It then goes into hibernation until March 6 (Michelangelo's birthday), when it springs into life and erases the C drive. Since it destroys itself in the process, it's incapable of doing further damage.
  
  

CIH (Chernobyl)

• The CIH virus first appeared in Taiwan in 1998 and quickly earned a reputation as one of the worst viruses the world has ever seen. It got its name from its April 26th trigger date, which is also the date of the Chernobyl meltdown. The CIH virus can hide undetected in memory and attack any applications on an infected machine. Depending on the version that infects a victim's computer, Chernobyl delivers its payload on April 26, June 26 or the 26th of every month and overwrites the booting hard drive. Worse yet, it's able to flash the BIOS (the computer boot-up instructions) of an infected computer to make it unbootable. Since the BIOS is usually soldered to the motherboard, CIH victim's are often forced to replace their motherboards to regain the use of their computers.
  
  

Magistr

• March 2001 was a really bad month for the antivirus community. That's when the most destructive virus they've ever seen began trashing computers. Magistr is in a league of its own. Its anti-debugging ability makes it hard to find and remove, and it spreads to other computers by mass-emailing every contact in a victim's email account. When recipients open the infected attachment, the real fun starts. After sitting idle for a month, Magistr delivers a payload that rewrites the host computer's CMOS (the computer system settings), erases its BIOS (the computer boot-up instructions) and destroys its primary hard-drive controller. When the dirty deeds are done, a vulgar dialogue appears, mocking the victim. Magistr can even make your desktop icons move away from your mouse. If there's a virus hell, this is it.