They say this virus has surged through more computers than anything they have seen in years.
The virus's coding suggests a type of bug that alerts computer users to bogus infections on their machines and offers to help by selling them antivirus software.
Instead, the virus is simply spreading to little effect, though it may still pose a threat to infected computers.
"The people behind the worm apparently haven't used it yet, 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.
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."
Most computers with Windows will automatically download Microsoft's security update but on affected computers the virus disables updates on infected machines.