How to Change Product Key without Reinstalling Your Windows XP
Say your Windows XP installation (already activated) somehow failed Microsoft Genuine Advantange Test (e.g. your legit product key has already been used by somebody else with Windows XP keygen). You can change the product key used to install your Windows XP without reinstalling your system. The basic idea is to deactivate your system and restart the activation process.
STEP 1 - run regedit, locate \HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\CrrentVersion\WPAEvents, change at least one digit of the value of OOBETimer to deactivate your installation of Windows XP. Close regedit and restart your system.
STEP 2 - run "c:\windows\system32\oobe\msoobe /a" to activate your Windows XP. Select Telephone option, then change product key, input new key, and confirm the change, and close the activation window. If you have used a VOL key, your system will show already activated after reboot.
STEP 1 - run regedit, locate \HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\CrrentVersion\WPAEvents, change at least one digit of the value of OOBETimer to deactivate your installation of Windows XP. Close regedit and restart your system.
STEP 2 - run "c:\windows\system32\oobe\msoobe /a" to activate your Windows XP. Select Telephone option, then change product key, input new key, and confirm the change, and close the activation window. If you have used a VOL key, your system will show already activated after reboot.