Repair XP and you cannot update to latest patches.

Microsoft Corp. Friday confirmed that Windows XP users who repair the operating system cannot update their PCs with the latest patches because of a file included with the stealth update pushed out to machines this summer.

"When an XP repair CD is used, it replaces all system files (including Windows Update) on your machine with older versions of those files and restores the registry," said Nate Clinton, program manager for Windows Update (WU), in a post to the Microsoft company blog dedicated to the update service. "However, the latest version of Windows Update includes 'wups2.dll' that was not originally present in Windows XP. Therefore, after the repair install of the OS, wups2.dll remains on the system, but its registry entries are missing. This mismatch causes updates to fail installation."

The Windows Secrets newsletter reported the patch installation failures after tests on Windows XP machines that had been restored by an in-place reinstall. The root of the problem, said the publication, is that seven DLLs from the latest revision to WU -- not just one -- failed to register themselves with XP. Microsoft could not provide an explanation for the discrepancy between the claims.

The file cited by Clinton, wups2.dll, is one of the seven fingered by Windows Secrets and part of the so-called stealth update that Microsoft sent to most noncorporate Windows XP and Vista users beginning in July and running through this month. The update was delivered and installed without prior notification, even when the PC's owner had told the operating system not to download or install updates without notification and permission.