Let me take a wild guess.
BSOD with "Unmountable_Boot_Volume" or something to that effect?
You cannot take a windows installation from one computer to another.
there is some small cases where i have seen it work with driver roll backs (while the HDD is in the old PC)
I'm guessing the OLD pc is WinXP and 32bit at that. I would not bother even if u do get it working ull only see 3 of ur 12gb of RAM
bite the bullet and reinstall.