im parting out 2 computers to make 1 good computer. the motherboard i decided to keep, had a 30GB hard drive with it, and it is what I am using now. On my other computer I have a 120GB hard drive I want to put in here instead.. But it has a different OS on it, and when I try to boot it, it tries to repair itself (in windows 7 ultimate) but it says it cannot repair it. Is it even possible to reuse this HD on a new motherboard? And if so, how do I do it?

Try installing it as a secondary (or slave) disk (not the boot disk). Then you should be able to format, partition, or access files.