The setup froze on product key entering screen, so i put my hard drive in
another computer to install vista onto it. This worked fine, and on THAT
computer Vista runs alright. But when i move the hard drive back to the
original computer and start loading it, it gets to the loading screen with
the loading bar, but then restarts. I've tried booting in safe mode, but same
problem.

From what i know and im not a pro its like this. You registered the key on
the one machine and it works fine. When you installed it Vista took inventory
of the machine it was installed on. So when you try to put it in another
machine it knows its not the same one and stops. It is like software you buy
the key is good for one machine, when you try to put it on another machine
and register it it will stop you and say its already been registered. You
have the same scenario here. One Key one machine. So you either erase the
hard drive and put it back in the machine you want vista on and reload it or
leave it in the machine you originally installed it on. Good luck.