Re: mac address
The mac address is hardwired on the motherboard.
But if it's just a question of Windows 7 failing to activate on a new install because of the new motherboard, call the special (free) activiation-issue phone at Microsoft, and they might be willing to give you a new license code for free.
That new install most like is necessary.
It won't harm to make a backup of the info on your hard drive before doing that reinstall of Windows. But if you use the original Windows 7 DVD it should save all old contents in a windows.old-folder.