Option 1: Refer to http://www.petri.co.il/change_the_serial_in_windows_xp.htm
Option 2: Follow the steps below:
1. Click on Start and then Run.
2. In the text box in the Run window, type regedit and click OK. This will open the Registry Editor program.
3. Expand folders until you reach the HKEY_LOCAL_MACHINE\Software\Microsoft\WindowsNT\Current Version\WPAEvents registry key.
4. In the results that appear in the window on the right, locate OOBETimer and Modify its value from the resulting menu. Change at least one digit in the Value data text box and click OK.
This will deactivate Windows XP.
5. Click on Start and then Run.
6. In the text box in the Run window, type the following command and click OK.
7. When the Windows Product Activation window appears, choose Yes, I want to telephone a customer service representative to activate Windows and then click Next.
8. Click Change Product Key and type your new Windows XP product key in the New key text boxes and then click Update.
9. Restart your PC.