, China and Argentina are the countries that have been hit hardest by the worm, which started spreading about two months ago but is thought to have infected millions over the past few weeks. China accounts for close to 29 percent of the infections tracked by Symantec, and Argentina was second with just over 11 percent, according to Alfred Huger, vice president of Symantec Security Response. "We're not seeing anywhere near the number of infections in western Europe and North America."

The worm, known by several names including Conficker and Downandup, has been spreading by taking advantage of a flaw in a Windows Server service that Microsoft patched last October. Conficker can also spread by guessing administrative passwords on a network and infecting USB devices that connect to computers.

Infection rates in the U.S. are closer to the 1 percent level, Huger said.

Phil Porras, program director at SRI International, said the worm has hit China, Brazil, Russia and Argentina the hardest. Interestingly, an earlier variant of Conficker would not attack victims who were using Ukrainian keyboards, but the latest version of the worm does.

Huger said the worm's designer has written special code that operates a certain way on Chinese and Brazilian networks, meaning those two countries may have been targeted by the attackers.