Kochis acknowledged that "some failures" are caused by users with genuine copies of Windows XP who improperly install or repair software on their PC. Such activities "will result in WGA validation failures, and they should," he wrote.

But Kochis also said that there are a number of other scenarios "that could result in a WGA validation failure that a user might be surprised by or even deny."

They include users unknowingly being sold copies of Windows XP by stores that illegally reuse the same license key with multiple customers, users who take their PCs for repair into shops who similarly reuse the same license key, users that share copies of Windows XP with their friends or acquaintances, and users who reuse the same key on more than one PC at a time.

Under Microsoft's strict licensing policy, users who bought a PC from a hardware vendor such as Hewlett-Packard Co. or Dell Inc. with Windows XP preinstalled typically own a reseller license that forbids them from installing XP on another computer -- even if the first PC is no longer functional. Microsoft wants users to go buy a full-priced retail copy of Windows XP for new PCs. Windows XP Professional currently costs $299 at CompUSA.com and goes for similar prices at other stores.

Kochis said Microsoft investigates all "credible" reports of genuine copies of Windows XP failing to validate under WGA.