Silly.Go,
I have two things for you to try. The first is the PKUT.
Thank you for contacting the Windows Genuine Advantage (WGA) program forum. The fact that you have a Microsoft Certificate of Authenticity for XP Professional affixed to your system indicates there is a strong likelihood your system is still licensed to run XP Professional.
The following link will assist you with detailed instructions for updating and using the Product Key (PK) Update Utility.
http://www.microsoft.com/genuine/selfhelp/pkuinstructions.aspx
Next you will be required to select the version of Windows currently installed on your system. The PK Update Utility was designed to update a non-genuine key previously used to activate XP and/or Vista with a known good product key. If you have antivirus, antispyware, or firewall programs installed, please make sure these programs are set to allow the product key update utility to have full access to the Internet so it may perform correctly.
If the above fails, please refer to the link below and run the Microsoft Genuine Advantage Diagnostic Tool:
http://go.microsoft.com/fwlink/?linkid=52012
By clicking the link below, you will download the Microsoft Genuine Advantage Diagnostic Tool. Then click the Run button. Then click the Run button again. Then click the Continue button to perform the diagnostic. The Windows tab should show, click the Copy button to paste your report to the clipboard. Then paste the report into a New Post on this Forum by adding a post in it and pressing Ctrl V (to paste the text). Clicking the OK button will clear the Diagnostic window. Here is the download link:
http://go.microsoft.com/fwlink/?linkid=52012
Step 2: Tell us about the Certificate of Authenticity (COA) for your copy of Windows.
1. What edition/version of Windows XP is titled? Home, Pro, or Media Center?
2. Does it read "OEM Software" or "OEM Product" in black lettering?
3. Or, does it have the computer manufacturer's name in black lettering?
4. Please provide the customer scenario you are experiencing.
5. DO NOT post the Product Key.
Not sure what to look for, see this page to reference your COA:
http://www.microsoft.com/resources/howtotell/en/coa.mspx
NOTE: The data collected with the Microsoft Genuine Advantage Diagnostic Tool does NOT contain any information that can personally identify you. The information can be fully reviewed, by you, before being posted.
Thank you for your participation here on the Windows Genuine Advantage (WGA) program forums. We look forward to working with you.
Rick, MS