I have a computer bought from a friend of mine. I follwed Microsoft suggestions how to change the WinXP Pro key product number (I have an original win XP pro license bought in 2002) - http://support.microsoft.com/kb/256986/IT
Now, when windowsupdate tries to validate my own original license, it is said that I have to buy a windows genuine kit.
I am a legal owner of an original cd rom sent me from Ireland (according to the info on cd rom package I receveid after the purchase of the license in an italian shop - cd discount network).
You may be a victim of software piracy. I would recommend either reporting it and submitting a counterfeit report (high quality counterfeit copies may be eligible to recieve a free WGA Kit) or purchasing a legal copy of Windows. Depending on your location, there are several methods described in your Validation pages.