Most common e-mail worms and viruses spread when the recipient opens the attached file, starting a program that infects the recipient's computer. The Mydoom worm, however, harbors its payload in a "zip" format, a compressed file that many corporate firewalls and anti-virus programs are designed to let through untouched.

The attached file -- which arrives as an innocuously named file such as "document.zip," "message.zip," or "readme.zip," contains a program that -- when opened -- immediately plants a "backdoor" program that lets the virus writer upload files to the infected machine.

Experts still have not cracked all of Mydoom's encryption code, which may hold clues about what else the worm is supposed to do.

Tony Magallanez, a systems engineer with San Jose, Calif., anti-virus software maker F-Secure Corp., said worm writers often use encryption to buy their creations as much time to spread as possible before experts can figure out what they are doing.

"The basic idea here is trying to make it difficult for the anti-virus researchers to stop whatever the worm is designed to do," Magallanez said.

Mydoom is already being compared to "Sobig.F," a worm that infected hundreds of thousands of computers worldwide, and later installed software that turned them into remotely controlled spamming machines.

Sobig spread at a rapid pace, giving the worm's author unrestricted access to computers infected with the worm.The computers were programmed to visit one of 20 Internet sites to download malicious software. An international team of law enforcement officials and virus hunters found and shut down those host Web sites hours before the infected army of hundreds of thousands of PCs were scheduled to follow their instructions.

Like Sobig, Blaster and most other viruses, Mydoom targets computers running the Windows operating system.