the copy is not genuine windows

If you swapped the motherboard, the old copy of windows is not technically legal any more.

With a fresh install and a call to ms you may be able to get them to activate, but by the rules, New board = New system and OEM copies of windows are tied to the first board they are installed on.

With the retail version you can move it from system to system as long as only ONE system has that copy on it.
 

MHRAHMAN

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matt_b

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If you are trying to say that this is what you did to an OEM computer, then the solution is written out as nukemaster said. Bets are you're buying a new key/license in the end.