1) Start Command Prompt, RIGHT Click it, and "Run as Administrator."
2) Key in the following:
slmgr.vbs -ipk ( here type in the key which you had - now deleted)
3) wait a while, maybe a few seconds or a little longer to deactivate Windows .
4)You will eventually see a pop up regarding deactivation, check and see if Windows is deactivated.
(Go to: Start> Right Click Computer> Click Properties, on the bottom it should tell you a time Limit)
You can now reactivate with your genuine key.