When I'm rendering an image in Poser or in Bryce that is larger the computer shuts off. It doesn't go through a shut down phase. The power light does not go off, but nothing responds and the heat sink fans stop. I have to hold down the power button to turn that off, wait a second and press it again to turn it on again. I thought at first there was a hardware issue but the guy at the computer shop says everything works fine. I reinstalled everything from windows up and have the update for poser.  I also bought a new battery backup (UPS) incase there was an electricity problem. Does this happen to anyone else? Is it a lack of memory issue? Do I need to find a new computer repair shop? Does any one have ant ideas, suggestions, wild guesses?

I have a PC with windows XP with a 2-gig processor, 1 gig of memory, and Poser 6.

get another GB or RAM?

Power supply or CPU overheating, I bet.

Take the fan off the CPU and clean out the heatsink vanes.  Clean the air slots in the power supply and just about everything else that you can find in there.  Get some more fans and shove them all over the place.

Do all this, of course, while your computer is switched off, else you - and it - will probably fry.

I tried cleaing the dust from it. do that everytime I open it  and had it running with out the cover for a while. Unfortunatly it didn't help.

I pull my heatsink once a month and clean it out.  I blow out the CPU fans every week, along with the 8 other fans I've installed.  This makes a big difference in performance.  Also, if you pull the heatsink, get some really good heatsink stuff ... my son works for Dell and he give me silicone jelly for my heatsink and says it's the only stuff to use.

As well as a heat issue this can be caused by a driver setting, there is an article on this very thing at microsoft knowledge base, have you installed any new drivers recently??

Ditto the overheating. The guy at the shop probably doesnt stress it like a 3d app does. Running without a cover doesnt help because the air is no longer moved through the case - imagine trying to heat your house by blasting the hot air in the basement without any ducts to transport the heat through the house. Now you could try running with a room fan aimed at the motherboard and see if that tells you it is indeed the heat causing it to shut off.  If it is - as pointed out - upgrade the cpu cooler and case fans.

I'm a bit rusty at the PC fixit biz - its been a long time - But from what I have noticed - cooling is even more important with these later generation rigs than it was when I made my nut fixing the things.

Good catch, Mystic.  Like you, my system is set to never hibernate because I've always got something rendering in the background so I didn't even think about that.

No.  I know it's not going into hybernation mode. I'll try the fan thing. Will t make a difference if I leave it off over night and then boot it up and try running the render right away? That way it has a chance to cool down?

Leaving it to cool off overnight won't matter much. The system won't retain the heat for very long. If you cool the system with a room fan, fifteen minutes cooldown should be enough.
The room fan idea is good, you'll force a lot of relatively cool air over the components.

By the same token, if it is a heat problem, the CPU will be scorching in a matter of minutes.

I encountered a similar problem in my office that hat me hitting my head against the wall for about the week, thinking it's overheating problem.
It turned out that the computer had automatic windows updateds turned on, and the machine set to automatically shut down and reboot when it was done (no prompting).
It finally gave me a clue when the guy complained that it was happening same time of the day, few days in a row. 
I don't think that's going on with your computer though.

Quote - the computer shuts off. It doesn't go through a shut down phase. The power light does not go off, but nothing responds and the heat sink fans stop. I have to hold down the power button to turn that off, wait a second and press it again to turn it on again.

Hatet to tell you this, but that was what was happening to my old computer in December and January.  It was overheating and finally just died. I couldn't get it to power on and stay on long enough to boot up in the end.  Plus all of the overheating ended up melting the motherboard.

aye aye to the overheating.  I had one way too old system with fans propped all around it in it's final days. 

Why is it that the one machine that is supposed to make life easier is the most difficult thing to work with? I had another heatsink so I put that in but to do so I ended up having to take everything out of my computer so I could take the motherboard out to get the old one out. I've taken other heatsinks out and didn't have that problem. Hopefully this will work.