first u hav to deactivate windows:
in regedit goto
HKEY_LOCAL_MACHINE\Software\microsoft\WindowsNT\Current Version\WPAEvents
then in the right pannel double click on 'oobetimer' and change atleast one digit of the value that comes up. u can change it to anything. this deactivates windows.
then go to run in the start menu and type
%systemroot%\system32\oobe\msoobe.exe /a
an activation screen should come up... choose activate by phone
the next screen has an option for 'change product key' enter ur new key in there.
reboot and all should be done.

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hope this helps