How to Change a Windows XP Product Key

Regedit

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.

   1. Hkey_local_machineSoftwareMicrosoftWindowsNTCurrent VersionWPAEvents
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%system32oobemsoobe.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 'You have successfully activated your copy of Windows.', if you do, restart your computer, and you're done!
12. 
    Make sure you are connected to the internet in this process!