this process is for changing the product key..it first deactivates the window then after you restart and finish process windows activates again..make sure you have your product key!!...i know this works for (xp corp).....hope this helps...
Change Product Key without Reinstall:
(1) Run regedit and go to: HKey_Local_Machine\Software\Microsoft\WindowsNT\Cu
rrent Version\WPAEvents,
on the right double click on "oobetimer" and change at least one digit of this value to deactivate windows.
(2) Choose run from start menu and type in this command:
%systemroot%\system32\oobe\msoobe.exe /a
to get the activation screen and go to the second option which is
activate by phone.
(3) In the new screen click button at the bottom that says change product key, and type in the new product key (Corporate one only).
then click the Update button.
(4) Close that window, reboot your system and repeat the command in step 2 for the second time to verify your
activation , you should see "Windows is already activated".