You used the old hard drive with the original operating system right?
If your OS was Windows 7 (vista too, I think)
Get the OS's installation disc, boot off of that, and click repair my computer.
Try to do something from there.
What happened was that your old operating system (Windows 7?) had a file that sort of matches itself with your motherboard.
When you switched your motherboard, that file (some .dll file) didn't 'recognize' your new motherboard and therefore would not let your OS boot.
Hope this helps.
How I know:
I had the same problem.