I had a similar problem with an IBM Thinkpad running XP Home. After reading around for solutions I installed a dodgy (warez) copy of XP Home to the laptop and failed the 'Geniune Windows' test on purpose. I then followed the instructions on the screen and entered the Windows Key on the bottom of the laptop into a box and voila - all activated and working.

This was a while ago, so I'm not totally clear on the details. I tried to find some better information but this page on WGA Failure Scenarios from Microsoft is the best I could find right now.