"The gang behind this worm haven't used it yet," F-Secure's chief research officer, Nikko Hypponen said by phone. "But they could do anything they like with any of these machines at any time."

Microsoft issued a security update Tuesday to deal with the so-called "Downadup" or "Conficker" virus, which appears to be a new version of a bug that popped up in October.

"Over the last couple of weeks, a new variant of this worm has been affecting customers," the company acknowledged in a blog post. Microsoft said the virus is spreading by gaining access to one computer and then guessing at passwords of other users in the same network: "If the password is weak, it may succeed."