An expert witness in the Vista class-action suit against Microsoft has claimed that the company made more than $1.5bn (£1.02bn) as a result of its Vista Capable marketing campaign.

According to documents released to the court, Keith Leffler, an associate professor in the University of Washington's Department of Economics, used Microsoft's own figures to estimate that the company made huge revenues from badging some computers as capable of running Vista before the operating system was released.

"I have been asked by plaintiffs' counsel to estimate the amount of revenue earned by Microsoft from the licensing of Windows XP on Vista Capable, but not Vista Premium Ready PCs sold to plaintiffs," his report states.

"In Microsoft's Supplemental Responses it estimates that it received revenue of [redacted] from Windows XP licences on upgradeable PCs sold in the US during the April 2006 through January 2007 period.

"From these figures I have therefore reached the opinion that Microsoft revenue from the Windows XP licensing on Vista Capable, but not Vista Premium Ready PCs sold to plaintiffs, was $1.505bn."

If accepted the figure could be used as a benchmark if the court finds against Microsoft.

The class-action suit alleges that Vista Capable computers sold before the release of the new operating system were capable of running only the most basic version of Vista, which the plaintiffs claim is virtually the same in function as Windows XP.

Microsoft has tried unsuccessfully to dismiss the case, or at least to have evidence kept out of the public record. The company has been embarrassed by a series of emails from its partners and own senior management lambasting the marketing programme.