When a computer will not fully boot...

[Pat D. Stephens]

I troubleshoot for a friend on his iMac and unknown to me, he had the 
logic board replaced a few months ago.  Since then it's done okay, but 
now today, it won't fully boot.  I went over and tried to boot from his 
Norton disk and it will only go about a fourth of the way up and then 
it freezes.  I've never seen this before and imagine it's way beyond my 
capacity to fix and he'll have to return it to the shop for a hardware 
fix, but do you all have any thoughts about what I can do first?  I 
have an external drive running a different operating system that I 
could use diagnostically perhaps (it's 9.1, and he's running 8.6), but 
if it won't boot past a certain point even from his CD Norton, would 
that be any good?

We are about an hour from the shop that works on Macs, so if there is 
anything I can do, I'd like to try.

TIA,

Pat