Use Regedit and find:
In the right pane, right-click the ODBETimer binary value and select Modify. Change at least one character of this value to either a number from 0 to 9 or to a letter from A to F, and then click OK and close the Registry Editor.
This renders the current product key invalid and deactivates Windows.
Reactivate Windows using your new product key.
Click Start | Run and enter the command: %systemroot%\system32\oobe\msoobe /a
Windows will launch the Product Activation Wizard.
Select the option to telephone a Microsoft customer service representative to activate Windows.
Select the Change Product Key option and enter your new product key.
Click Update and close the window.
If Windows returns you to the previous screen, just select the Remind Me Later option. When the wizard is finished, reboot the system.
When Windows reboots, your next step will depend on which Windows XP version you are using. If you have XP Home or Professional, you’ll be prompted to reactivate your copy of Windows through the normal activation process. If you have XP Corporate, no activation is required and your machine should have a valid product ID.