1. Run regedit and 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.
2. Choose run from start menu and type in this command:
%systemroot%\system32\oobe\msoobe.exe /a ( where %systemroot% is most likely C:\WINDOWS\ ) to get the activation screen and go to the second option which is activate by phone.
C:\Windows\system32\oobe\msoobe.exe /a
3. In the new screen choose the option to change product key, and type in the new product key ( corporate one only ).
4. Close that window, reboot your system and enter the command in step 2 for the second time to verify your activation , you should see "Windows is already activated".