If you are running an existing installation of Windows XP and need to update or change the key—perhaps to legalize a "not genuine" notification—you can use the (often referred to as PKUT). Windows Product Key Update Tool - Bleeping Computer
If your PC originally came with XP, the key is often embedded in the BIOS (SLIC) or on a sticker. Reinstalling with the correct OEM disc will auto-activate. windows xp product key k2kb2 upd