You may How to Change the Product Key in Windows XP SP3:
Method 1: Change the Product Key Using Windows Product Key Update Tool
- Download and install the Windows Product Key Update Tool.
- Launch the Windows Product Key Update Tool.
- Enter your valid Windows XP product key and then click "Next."
- Select "Restart Now" and click "Finish" to complete the process
Method 2: Manually invalidating
Invalidate the existing Windows serial number
- Click Start, and then click Run. In the Open box, type Regedit, and then click OK.
- In the left pane, locate and then click the following registry key:
HKEY_LOCAL_MACHINE\Software\Microsoft\WindowsNT\Current Version\WPAEvents
- In the right pane, right-click OOBETimer, and then click Modify.
Change at least one digit of this value to deactivate Windows.
Enter the new Windows serial number
- Click "Start" and go to "Run." Enter the following string:
%systemroot%\system32\oobe\msoobe.exe /a
- Click "Yes, I want to telephone a customer service representative to activate Windows" and then click "Next."
- Click "Change Product Key."
- You will now be able to enter the valid serial number for your Windows XP.
- Press "Update" and close the window.
- Now restart your computer. Your version of Windows XP should now be updated with the new serial number.