When I installed the original hard drive, I ran the Windows xp home disk to 'repair' the OS for the new components, and when the repair was completed, I restarted the PC.
I think you were doomed from the start on this one. The only thing you can do now is call Microsoft and see if they will let you activate it.
You've replaced too many parts. Even if you can get this OS to start up, you won't be able to activate it. I don't know the exact policy regarding motherboards, but I think they frown on using a different board than you first activated with, on the sublime theory that the motherboard is the computer and you're now, in effect, trying to re-install the same OS on a "new computer."
You can read more about product activation Here
and if then your best bet is probably to call Microsoft. Sorry, I wish I had better news.