Smallvoid.com

It is possible to change the WinXP CD Key without reinstalling completely.

1. Run System Restore to create a new Checkpoint. So that you can rollback if something goes wrong
2. Start regedit and change at least one digit of the "oobetimer" value :

   [HKEY_LOCAL_MACHINE \SOFTWARE \Microsoft \Windows NT\CurrentVersion \WPAEvents]
   oobetimer = ? (Change at least one digit)
   

3. Start the Windows Product Activation (WPA) screen by running this command :

   %systemroot%\system32\oobe\msoobe.exe /a

4. At the activation screen, select the option to "Activate by Phone"
5. At the product key change screen, type in your new product key (Make sure to use a key which match your Windows license)
6. If it returns to the activation screen, after typing in the new key, click "Remind me later"
7. Close the window and reboot the machine
8. After the reboot run the command to start the Activation Screen again and make sure that it says "Windows is already activated"

Note the new CD-Key has to match the license of the installation. If having installed a WinXP Pro Corporate Edition (Pirated Version), then the new CD-key must be a corporate CD-key (And not a Retail or OEM CD-key).

More Info MS KB Q328874
More Info MS KB Q842196
More Info MS KB Q918342

Related Change WinXP license before installing
Related Retrieve a lost CD-Key from registry
Related Save the WinXP Product Activation before reinstall

Credits Bink.nu