Normally you need to reinstall windows after motherboard change. What you may try is is to perform a startup repair with the win7 disk. Most of the time it will be able to boot afterward.
I suggest you backup your files as soon as you able to boot in windows 7 and then reinstall windows 7 from scratch. Motherboard change without full windows reinstall normally leads to an unstable computer.
Here is tutorial about this :http://www.sevenforums.com/tutorials/681-startup-repair.html