One way that I've done this in the past is to pop the motherboard CD in, for the new motherboard, BEFORE you change the hardware out. Install all the drivers for the new mobo first. Then shutdown and change out the mobo. If you're lucky it'll come up with the new drivers and start yelling about how Windows isn't genuine anymore
If you're unlucky it just won't come up and you'll have to reinstall.

If you're unlucky it just won't come up and you'll have to reinstall.