How to Enter a Genuine XP Product Key Without Reformatting
- 1). Visit the Genuine Microsoft Software page and read the instructions for how to use the "Windows Product Key Update Tool."
- 2). Click the check-box to agree to the statement "I understand this tool will send the above information to Microsoft." Press the "Download" button when it becomes available.
- 3). Choose "Run" or "Open" when your browser tries to download the tool. Type your genuine product key into the five boxes on the "Your Product Key" screen. Hit "Next."
- 4). Push "Finish" when the update process completes. This will automatically reboot your computer and finalize the changing of the product key.
Source...