My newly reinstalled imac g5 (isite) crashes/freezes after first lot of updates

I got this from a someone last week, I erased & reinstalled it, it worked fine until I did the first lot of auto updates - after it restarted it lasts for about 30secs until freezing. The only way to shut it down is to hold power key. When trying again, it continues to do the same thing.

I have re-installed 3 times now.

I did a hardware test I saw recommended which comes back fine, so im assuming its a software/update issue?

The previous owner said it was fine the whole time they had it, we're both guessing that could be because of the updates.

Could it be that it is updating 2005 to 2011 in 1 big step? What can I do? Can I update small steps instead and how far can I update to without wasting time/multiple downloads only to find it crash again? How would I do this? Any help would be much appreciated :S! thanks, bess