More than one in every three new PCs is downgraded from Windows Vista to the older Windows XP, either at the factory or by the buyer, according to a US performance and metrics researcher.

Devil Mountain Software said nearly 35 percent of the 3,000-plus PCs it examined had been downgraded from Vista to XP.

"Either these machines were downgraded by [sellers like] Dell or HP, or they were downgraded by the user after they got the machine," said Craig Barth, the chief technology officer of Devil Mountain. "In any case, these machines are no longer running Vista."

Barth used data provided by users to Devil Mountain's - which it kicked off last year and has expanded by partnering with Infoworld, a PC Advisor sister publication - to come up with his numbers. By collating such things as the vendor and system model number with manufacturers' catalogues, Barth was able to identify machines that were probably shipped in the last six months, a period when virtually every new PC was offered with Vista preinstalled.

"The 35 percent is only an estimate, but it shows a trend within our own user base," Barth said. "People are taking advantage of Vista's downgrade rights."

How to downgrade from Vista to XP

How to extend Windows XP

By the terms of Microsoft 's end user licensing agreement (EULA), Vista Business and Vista Ultimate can be 'downgraded' to XP Professional; businesses that purchase Vista Enterprise can also downgrade to XP.

Although Microsoft retired Windows XP from mainstream availability at the end of June - it stopped shipping the seven-year-old operating system to retail and large computer makers - some OEMs have continued to offer new PCs with XP preinstalled by doing the downgrade at the factory.

"Vista's installed base certainly doesn't equal the number of Vista licences [that Microsoft's] sold," Barth said, citing the data as proof. "We're seeing this a lot in the financial sector."