what orlbuckeye said is correct - you can install the OEM windows copy again but you'll constantly get msgs that this is an invalid copy or possibly a counterfeit copy of windows
i had the same problem when i changed motherboards (running win 7 x64 OEM copy) and when i called into microsoft, i indicated my motherboard had fried and I was simply replacing a bad mobo - they gave me a key for a one time use -
hope that helps and fwiw, surprisingly it didn't take that long on the phone with microsoft to get thru and get the assistance