Note We recommend that you run System Restore to create a new restore point before you follow these steps.

Deactivate Windows

1. Click Start, and then click Run.
2. In the Open box, type regedit, and then click OK.
3. In the navigation pane, locate and then click the following registry key:
   HKEY_LOCAL_MACHINE\Software\Microsoft\WindowsNT\Current Version\WPAEvents
4. In the topic pane, right-click OOBETimer, and then click Modify.
5. Change at least one digit of this value to deactivate Windows.

Reactivate Windows and add new product key

1. Click Start, and then click Run.
2. In the Open box, type the following command, and then click OK.
   %systemroot%\system32\oobe\msoobe.exe /a
3. Click Yes, I want to telephone a customer service representative to activate Windows, and then click Next.
4. Click Change Product key.
5. Type the new product key in the New key boxes, and then click Update.If you are returned to the previous window, click Remind me later, and then restart the computer.
6. Repeat steps 1 and 2 to verify that Windows is activated. You receive the following message:
   Windows is already activated. Click OK to exit.
7. Click OK.
8. Install Windows XP Service Pack 1a or a later version of Windows XP.

More info : http://support.microsoft.com/kb/328874