Do you have Windows XP? Is the "Inaccessible boot Device" a windows Blue Screen or such error or is it a POST error? If it is a Windows or Such Error, I would boot off the CD to the recovery console and run FDISK /r to try to repair errors. You could even run FIXMBR if it still has problems to repair the Master Boot Record.

If it is Windows XP, the new motherboard will cause Windows XP to Blue Screen if IDE is not identical on both motherboards and will require a reinstall (Windows XP is not as nice as Windows 98 or even 2000 for that matter).