Last year I upgraded my Windows 98 PC with a copy of Microsoft Windows XP SP2 Professional I bought online from a company that has since gone out of business. At the time I verified the software's validity using Microsoft's Windows Genuine Advantage site, which I had to do in order to download and install Windows Media Player and the beta of Microsoft AntiSpyware.

However, a month ago, when I tried to upgrade to the shipping version of the antispyware software, my copy of XP failed the WGA validation test. Why would my copy of XP have passed the validation test once and then failed?

Billie J. Osborne, Whites Creek, Tennessee

On Your Side responds: A spokesperson for Microsoft says that while Osborne's plight is rare, it does occur with pirated software that has a counterfeit Product Activation key. The validation process, which happens whenever a Windows user requests a software download, checks the user's key against those in Microsoft's database of counterfeit keys. Since Microsoft is constantly finding new counterfeits, the results of the validation check can change.

Microsoft recommends visiting the WGA Web site to learn how to recognize fakes, and, if necessary, to fill out a report of a counterfeit. Some customers may be eligible for free replacement software.

Amber Bouman is an editorial assistant for PC World. E-mail her at onyourside@pcworld.com. Find links to previously published On Your Side columns below.