How to Change Product Key on activated WinXP machine (OEM)

1. Open registry Editor, Change HKLM\Software\Microsoft\WindowsNT\CurrentVersion\WPAEvents\OOBETimer by one (at least)  character.
2. Search C:\Windows for WPA*.* and delete the wpa.dbl & wpabaln.exe files
3. Execute START=>RUN; "%systemroot%\system32\oobe\msoobe.exe" /a then press OK
4. Select ACTIVATE BY PHONE
5. Select CHANGE PRODUCT KEY
6. Enter in new valid key and press OK
7. Go BACK to the first menu
8. Select ACTIVATE OVER INTERNET
9. Register if you want to
10. Success!

http://www.tomshardware.com/forum/86029-45-change-product-winxp-machine