Hardware that runs Microsoft's Windows Genuine Advantage (WGA) system crashed for around 19 hours over the bank holiday weekend causing havoc for thousands of legitimate Vista users.

They not only experienced reduced functionality mode such as the Aero graphical interface being disabled because validation failed but also found themselves 'accused' of running pirate copies of the operating system on their PCs.

The problem which lasted around 19 hours was blamed by Microsoft on hardware problems. Not surprising many Vista users were left seething, leaving increasingly colourful comments about the issue on Microsoft forums.

Phil Liu, a Microsoft program manager in the Windows Genuine Advantage group, said the cause of the problem, which was finally fixed on Saturday, is still not entirely clear. "I know you all are looking for an explanation/root-cause," Liu wrote. "We are aware it is a server-side issue - the cause is unknown at this current time."

If your computer is still reporting an invalid license, Microsoft suggests you go to the WGA website and revalidate.