Click 'Start' => 'Run'

In the 'Open:' box, type regedit

navigate to: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\WPAEvents

In the right window, right-click on OOBETimer and select Modify

Change one of the digits in the window and then click 'OK'

Close the registry editor.

Click 'Start' => 'Run'

In the 'Open:' box, type      %systemroot%\system32\oobe\msoobe.exe /a

Click 'Yes, I want to telephone a customer service representative to activate Windows', and then click 'Next'

Click 'Change Product Key'

Enter the new product key

Click 'Update' (If the previous window opens, click 'Remind me later')

Restart your PC