Some Windows XP machines are vulnerable to the Zotob worm that ran riot on Windows 2000 systems last week, Microsoft has said.

In a "clarification", Microsoft said that PCs running Windows XP Service Pack 1 are also at risk if a file-sharing feature called “Simple File Sharing and ForceGuest” is enabled. The company hadn't seen any attacks trying to exploit this scenario, it said.

Zotob made headlines last week when it infected systems around the world. The virus, which has since appeared in more than a dozen variants, exploits a flaw in a plug-and-play feature of Microsoft's operating system and causes infected machines to continually reboot, among other things.

Security experts have called it the worst virus outbreak so far this year, although the rate of infections has been falling since late last week as more users patch their systems. "It's definitely going down all the time," Mikko Hypponen, chief research officer at anti-virus company F-Secure said.

Most Windows XP users have probably upgraded to Service Pack 2 by now, so the risk to XP machines is probably not very significant, he said. There are now more than a dozen variants of Zotob, at least one of which is spreading via e-mail, which makes it easier for the virus to get behind corporate firewalls, Hypponen said. Users are advised to install the patch Microsoft issued two weeks ago and keep their anti-virus definition files up to date.

Microsoft acknowledged in its original security bulletin that some Windows XP machines could be at risk, as well as systems running some versions of Windows Server 2003. But originally said to exploit those operating systems a virus writer would need a valid user log-in.

But where the Simple File Sharing Feature is enabled, other users can access those PCs using a "guest account," for which a log-in is not required.