Any site ONLY selling you a code for windows 7 is a scam site.
These sites find keys (usually ones purchased by large businesses/governement that were not yet activated). So the key you have is illegal and you dont actually own it, thus if it ever is activated by its rightfull owner your copy is deactivated and left high and dry.
Buy from an authorized microosft resealer like newegg, amazon or your local computer store.