You will need to re-install, and probably buy, Windows.
In my experience Windows just outright cant boot when you try to run it on different platform than what it was installed on, and a different mobo and CPU is a pretty big change.
Even if Windows did recover to the hardware change, you would have an invalid copy as an OEM key is only valid on one system (the retail version is good for up to three machines I think). Microsoft defines a system by its motherboard, your Windows key has tied itself to that motherboard and is only good to use on that mobo (again, retail version being good for up to three).
Until you punch in a valid key, you would have an invalid (and possibly illegal) copy of Windows.