After having my legit Windows XP Pro come up with an WGA nag screen, only to be told by Microsoft the only way to repair this was to reinstall XP pro (no thanks). I decided to hack it instead only to find MS actually has the fix themselves (DOH!). Just be careful not to reinstall the nag screen yourself afterwords, and I suggest method 1 only.
Tech Net TV - 
I am not having any luck bypassing the validation thing, esp trying to update to Internet Explorer 7….please help
Excellent info on how to bypass wga and/or remove and prevent wga and make your copy of windows xp pro validate successfully across the board. Including via Windows Update.
http://spacebag.googlepages.com