When installing Windows 7 it's very picky about which installation disc you use (Full vs. Upgrade) and if improperly matched with your key can prevent you from activating your copy of Windows usually returning an "Invalid Product Key" error message. You may also see the same or similar error when you do a full, clean installation with an upgrade disc. This can be very annoying, especially when you have a legitimate key and disc but just didn't install it the way Microsoft thinks you should. In the event that you are having trouble activating your Windows 7 installation with a legitimate key, try the following registry hack: