How to change the serial number used in Windows XP, Valid for XP Corporate:

Step 1

Open Start/Run... and type the command:
regedit and click "OK" (or press ENTER).

Go to HKey_Local_Machine\Software\Microsoft\WindowsNT\Current Version\WPAEvents,
on the right double click on "oobetimer" and change at least one digit of this value to deactivate windows.

Click "OK" and close the Registry Editor.

Step 2

Open Start/Run... and type the command:
%systemroot%\system32\oobe\