Hi Mark...........long time no see?

I guess your first move is to see if this is hardware or software?.....after all:

New Hard Disk (ghosted from old one)
So you just might have ghosted the problem?

My first move would be to get any sort of boot disk and fire her up with that..........if she doesn't shut down, then you have an OS/software issue.

You don't say, so I will guess Windows XP, or at least Win 2000; so is there anything in the logs?

Also, will she fire up and stay running in safe mode?

This is in addition to what has already been suggested

Cheers,

Johnno