A bug in Windows Vista's built-in antipiracy technology is telling some users that they need to reactivate the operating system after they install new device drivers or run newly installed software.

Microsoft quietly issued a patch designed to fix the flaw in its Software Protection Platform (SPP) technology late last month. Criticized by some when it was announced last fall, SPP is an updated and more aggressive version of the Windows Genuine Advantage antipiracy tools that Microsoft included with Windows XP. But because of the bug, SPP may suddenly demand that a copy of Vista be "activated" even though the user and/or the computer maker did so earlier.

"You may be prompted to activate Windows Vista on a computer on which Windows Vista activation was not previously required," Microsoft said in a support document last updated Feb. 15. "Although this problem rarely occurs, it may occur during typical use of a Windows Vista-based computer. For example, this problem may occur under one or more of the following conditions: You install a device driver, you install a program, you run a new program, you remove a program."

One Vista user commenting on Microsoft's Windows Vista Validation Issues support forum reported that he ran into the bug after he had updated his PC's BIOS.

Vista must be activated within 30 days of its first use, or else it drops into what Microsoft calls a "reduced functionality" mode -- a crippled condition in which only the operating system's Web browser works, and then for only an hour at a time.

Unless a PC's hardware is substantially changed, activating Vista should be a one-time chore. Many hardware vendors preactivate the operating system for users before their machines leave the factory. But if users are asked to reactivate Vista, they only have a grace period of three days, not 30, to get everything on their systems shipshape.

Microsoft said the reactivation flaw crops up when a program running with administrative credentials removes a system setting. That in turn can trigger the failure of a BIOS validation check, which is part of the activation process. For example, running Intuit's QuickBooks 2007 software can bring up the reactivation demand, Microsoft added. But, it said, "this problem may also infrequently occur when you install other programs or device drivers."

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