I purchased a new HP Pavilion d4650y desktop PC last November. The computer ran Windows XP Media Center Edition, but came with a guarantee that I would receive an upgrade to Windows Vista Home Premium once it was released. In December, HP provided a Web address where I could apply for the upgrade kit. When I applied for the upgrade, however, I kept getting a notice that my Certificate of Authenticity, a registration number proving that I had bought the computer, was not recognized. I contacted HP's customer service department, and was told that the problem was Microsoft's. When I called Microsoft, it said the fault was with HP.

Palmer Gilleland, Spring Branch, Texas

On Your Side responds: After we contacted both companies, a representative with ModusLink, an HP contractor, assisted Gilleland in completing the upgrade application. At press time an HP rep said Gilleland should receive his copy of Vista within four to six weeks.

HP gave this explanation for Gilleland's problem: When customers entered their COA number on the company's Web site, that number was blocked from future use, as a security measure. If the application process got interrupted, however, the COA number could have been locked before the application was complete. The application process has since been changed to fix this problem, the HP representative said.

Amber Bouman is an editorial assistant for PC World. Send e-mail to onyourside@pcworld.com.