By "hack" I am again assuming again, wow I do that alot, that you are using magic jellybean keyfinder which is a great utility! It could be that your key has been activated one to many times, which is fine and normal. It really depends on the style of key how many times you can activate before it buzzes at you. Some are one or two activations some are indefinite. Put your real, valid, key in and try to activate, it will probably say invalid which is fine so select the validate by phone option. Chances are you will talk to a computer the whole time. Read off your activation code and confirm that it is installed on ONLY THIS computer. It or a person, if the computer can't activate which is again normal, will then read off an activation code which you type in and away you go with a valid Windows installation.
Now that I'm thinking about it yours might not be activating because it is an OEM copy and your rebuilt laptop does not match the OEM specs that go with the Windows Installation Copy. So in all reality it could be perfectly fine and under the activation limit just with a little problem.