Windows XP can also be hit by the recent spate of Windows 2000 viruses, it has emerged.

Microsoft revealed on Tuesday that computers running with Service Pack 1 have the same vulnerability as those with Windows 2000. Although Microsoft has assured customers that it is very unlikely attacks to XP SP1 machines will occur, antivirus companies have already seen examples.

"I believe we did [see examples of infection] with Zotob.C," said Mikko Hypponen, director of antivirus research at Finnish antivirus company F-Secure. "It spreads also via email, not just the PnP exploit."

Zotob and similar variants caused a spate of infections last week in what some industry experts are calling the worst virus outbreak this year. Last week SC reported Zotob had hit a number of media outlets, including CNN, the ANC network and the New York Times. Even some educational establishment, such as the University of Helsinki, were infected.