If the Klez worm was a movie, it might squash Spider-man. If it was a musical act, it would likely best Britney.

But it's neither. Klez is an insidious snippet of computer code that's rocketed to the top of the virus charts by spreading its payload via e-mail and vulnerabilities in Microsoft products. Its purpose: Like a real, living virus, it exists to replicate itself and, with some strains, to create a little havoc along the way.

It was first detected in October, according to April Goostree, virus research manager at McAfee.com, which provides online virus detection services and anti-virus software. Nine months later, Klez remains No. 2 on McAfee's virus threat list, and Goostree said that 12 percent of McAfee users have detected or been infected with the virus since it was discovered.

At its peak, 3,000 customers a day sought help in stopping Klez from the Symantec Security Response, an arm of the company that produces the Norton AntiVirus series of detection products. That's double the number of submissions that came from the Sircam virus scare, said Dee Liebenstein, the response team's product manager.

"Klez remains very widespread. It's still the No.1 virus by the number of user submissions," Liebenstein said. "We still receive 10 times the number of Klez submissions as even the next closest virus."

Why is Klez so widespread? In part because the distribution system is so simple. The worm can spread inside an e-mail attachment, or even as part of the e-mail message itself. That means, Liebenstein said, that merely opening an infected message can infect a PC running Outlook or Outlook Express.

That leads to the second reason the virus is so widespread: Not enough users run — or properly run — virus scanners on their PCs. A good scanner should be able to check your system for infections, as well as monitor the programs you run, the files you download, and the e-mail messages you receive. And it needs to have up-to-date virus definitions — the key files that enable the scanner to detect new viruses — to do its job.

"It's really important to emphasize the need to update your virus software," Goostree said. "It the only way to eradicate or slow down this virus."

The Klez worm hasn't received the notoriety of some of its viral cousins, like Melissa, Michelangelo, Iloveyou, or Sircam. That's likely because they're more destructive. Klez doesn't destroy data. Its most dangerous threat is that some variants sometimes pluck random files off your PC and sends them out as attachments. If it's sensitive data, like credit card numbers or banking information, you could create a security nightmare for yourself.

How do you prevent spreading Klez? Liebenstein suggests securing your system with a good virus scanner and up-to-date virus definitions. A good firewall program also is essential for cable modem and digital subscriber line (DSL) users whose machines might be exposed to hackers who could plant bugs or other malicious programs.