Hello psicoltd,
First, try the Product Key Updater Utility:
http://go.microsoft.com/fwlink/?linkid=45668When it asks for the new Product Key, use the PK that came in the XP Pro retail upgrade package, printed on a yellow-orange label.
If no joy, please run this utility:
http://go.microsoft.com/fwlink/?linkid=52012. Click the Copy button to copy to clipboard, then come back here, make a reply, and paste the full report into the reply.
For great advice on all topics XP, visit http://www.annoyances.org/exec/forum/winxp