A problem with Microsoft's Windows Genuine Advantage (WGA) over the weekend caused some legitimate copies of Windows XP and Vista to be labelled as fraudulent.

A company spokesperson told vnunet.com that the problem lasted for 20 hours, from Friday afternoon into late Saturday morning.

The company is still determining the exact cause, although marketing general manager Joe Williams attributed the problem to a server failure.

During the 20-hour outage, users who attempted to revalidate their copies of Windows were told that their software could not be validated.

For Windows Vista users, failing the validation check puts the system in a reduced functionality mode, disabling the Windows Aero interface, the Ready Boost cache utility, and non-critical security procedures in Windows Defender.

Windows XP users are presented with recurring dialogue and balloon notifications on failing the validation check.

Microsoft requires users to revalidate their systems after major upgrades, such as the replacement of a hard drive.

On a posting to Microsoft's WGA blog, senior product manager Alex Kochis said that the problem had affected "less than 12,000 users" and that the system was now fully functional and providing accurate validations.

Kochis also noted that none of the users was sent into Vista's 'Out of Grace' mode, in which pre-installed games and premium features are disabled and sessions are limited to one hour.

Kochis said that users can now revalidate their copies by visiting Microsoft's W GA validation website.

Some Windows users, however, were not immediately willing to forgive and forget. "I was and still am quite mad at Microsoft," a user by the name of 'michaelosity' commented on the Microsoft blog posting.

"I paid well in excess of $200 for my copy of Vista Ultimate and getting the 'Your copy of Vista appears to be counterfeit' and having functionality removed and not being able to install updates from the MS site doesn't make for a very good experience."