How to Change a Windows XP Product Key

http://www.wikihow.com/Change-a-Windows-XP-Product-Key

This article describes how to change the Windows XP product key after a Volume Licensing installation. You can use the Windows Activation Wizard graphical user interface (GUI) or a Windows Management Instrumentation (WMI) script. The Activation Wizard method is easier, but if you must change the product key for multiple computers, the script method is better.

1. Click start in the bottom left corner, then click run.
2. 
   Type 'Regedit' in the white text field, and click OK. This should open the Registry Editor.
3. Navigate to the following registry key.
   • Hkey_local_machine\Software\Microsoft\WindowsNT\Current Version\WPAEvents
4. 
   Right click 'OOBETimer', in that registry key that you just navigated to and then click 'Modify'.
5. Change one or more digits to anything random.
6. Click start and then click run.
7. Paste '%systemroot%\system32\oobe\msoobe.exe /a', in the white text field,.
8. 
   Select 'Yes, I want to telephone a customer service representative to activate Windows', then click next.
9. Click Change Product key.
10. 
    Type the new product key into the small white text fields and then click update.
11. Check that you get a message similar to 'Thank you for activating Windows XP', if you do, restart your computer, and you're done!