Put in a new drive, do a fresh install - it's the 'cleanest way'.
Plug in your old drive and move your data back.
There is a dodgy registry hack, but strongly not advised - been there, done that etc.
Edit: Just a heads up: your old product key won't be valid any more for activation; a new motherboard counts as "a significant hardware change" - quite rightly so! So you will have to phone up Microsoft to activate + explain what happened.
- There is no such word as "can't" -
- 60% of the time it works every time -