Microsoft claims software pirates prefer Windows XP to Windows Vista.

Bonnie MacNaughton, a senior lawyer at Microsoft, said: "Historically, counterfeiters tend to focus on the 'n-1' version of software. Because of the more robust antipiracy and security features in Vista, most sophisticated piracy rings still continue to focus on XP. But that's changing over time."

That pirates have stuck with XP - which retains the bulk of the Windows operating system's market share - is "very consistent with what we've seen in counterfeiting in the past," said MacNaughton.

"There's usually a lag of between one and two years [before they can] figure out how to replicate those antipiracy and security features."

Counterfeiters currently copy Office 2003 rather than the newer Office 2007 for the same reasons, she said.

"As counterfeiters have got more sophisticated, we have realised that this is not a situation that we can address alone," she said. "And we want to stress the collaboration with Microsoft's partners and customers, and governments."

In the US, Microsoft filed 20 new lawsuits in federal court against software resellers that, according to the company's allegations, either sold pirated copies of Microsoft Windows XP Professional and Office or installed the counterfeit software on new PCs.

"We're expecting that counterfeiters will attempt to fill the void at XP's end of sales," added MacNaughton.

Microsoft will halt Windows XP Professional sales to small computer sellers after January 31, 2009. Larger computer manufacturers such as Dell and HP, however, will be able to obtain XP media for 'downgrades' from Vista Business and Vista Ultimate licenses through the end of July 2009.

According to MacNaughton, Microsoft will roll out a campaign in early 2009 that will remind people of XP's demise and warn them that copies they obtain after those end-of-sale dates could be counterfeit.

"We're planning [a campaign] in January or February to make sure our customers know what our rules and policies are about Windows XP, to make sure they understand what may be illegitimate and what may be legitimate," she said. "We want to make sure that the XP they might be getting is genuine."