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#11
You can reinstall either bit version with any Win7 license. The installer and steps are in Clean Reinstall - Factory OEM Windows 7.
I would try reactivating on the new hardware. You haven't said how long the OEM ran before the mobo failed but the shorter the time the more likely they'll allow you to move the key to a new mobo.
And as I said we have repeat reports that MS makes exceptions when asked during the activation process to allow retail OEM to migrate. Kari reported about a year ago they allowed him to move several in a year's time.
Let us know what happens. Kari might have some pointers for you too since he has been able to do it every time he asked.