STOP.......
You can not "reuse" and old copy of Windows 7 with a new motherboard for two (2) reasons:
- 1st, it's likely to not even boot as it is configured for the original motherboard it was connected to...
- 2nd, it's license key is already in use for that old motherboard and you can't reuse it (unless it's a Full-Retail version...)
So, buy a new copy of Windows 7/8....
Reload the system from scratch using the new copy.... (if you just want to "test it" you can reload old Windows 7 from it's disks and it will work UN-registered for 30days...)
NOTE: I never recommend re-using an old hard disk in a new system, it's likely going to be the 1st thing to fail as it's already "old"...