I have recently come across a situation in which a computer would not allow me to use the traditional methods of activating windows 7. Here is a method which worked for me.
1. Open an elevated command prompt. (click the start menu, click all programs, click accessories, right click command prompt and choose run as administrator.)
2. Now in the command prompt type: slmgr.vbs -ipk (your product key). *note do not include the parenthesis but do include the dashes in your key.
3. You will see a prompt stating your product key has been successfully changed.