You can deactivate WPA through the registry, and then re-enter a different product key through MSOOBE.exe
I'll assume your being honest and really have purcahsed a genuine copy of WinXP as you stated.
Instructions-
RegEdit:
HKey_Local_Machine\Software\Microsoft\WindowsNT\Current Version\WPAEvents
Double click on OOBETimer and remove the "CA" digits on the top line.
Exit the registry and your copy of Windows will no longer be registered.
Run MSOOBE.exe, which should be located at
%systemroot%\system32\oobe\msoobe.exe
Go to the second option that says "Telephone Customer Service to Activate Windows".
Click on "Next".
On the botton of the applet will be a button that says "Change Product Key", enter in your new product key and click "Update" then reboot the sysem and your done.
Remeber it must be the appropriate Key for your version of Windows.
If you've ever disabled WPA functionality in the past then you will have to re-enable it before MSOOBE.exe will operate.
To do so you can download a piece of software such as XP-Antispy and re-enable licdll.dll
As always if your not familiar with the operation of the Windows Registry and WindowsXP's WPA software then I stringly recommend you contact Microsoft Technical Support. After confirming your valid purchased Windows Product key they can walk you through the procedure step-by-step.