It could be a number of things at this point. Putting the HD in another computer doesn't always work because of the different drivers and what not. Is it Win 7 or vista? If so, put the cd in and boot to it. Choose to do the startup and repair from there. You could also try to do a system restore too. I would also run (from the command prompt) chkdsk C: /r You could also download Ultimate boot cd and run some malware and virus scans just in case. If you can get the actual error that would help a lot too.
I have Win 7 pro and they use XP Home.