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.
4. In the command prompt type: slmgr.vbs -ato which will initialize the windows activation.
5. Once the activation is complete you will see a prompt notifying you whether this process was successful or encountered a problem.
If successful your product should now be activated and the little prompt at the bottom right will go away after a reboot.