An unknown problem in a controversial Microsoft Corp. anti-counterfeit process has been fingering genuine versions of Windows Vista and Windows XP as fakes since Friday night, disabling features on potentially millions of machines.

Beginning Friday around 8 p.m. Eastern, users started posting messages on Microsoft's support forums, including Vista Validation Issues, saying that their PCs had been tagged as running non-genuine Windows. Overnight, the number of users adding their accounts to the tale spiked significantly, and the frustration index jumped dramatically.