Since you typexd your whole question into the topic subject box, a bunch was cut off and it really doesn't say what your problem is, but more than likely, you will have to use your windows disc and reinstall.
Now with some versions of Windows 7, technically, your not allowed to change motherboards since that constitutes installing on a new PC, and it may not activate. Now if you installed a new motherboard because the old one died, then usually a quick call to Microsoft activation will get you reactivated.