Make certain you have plenty of cooling fans and your CPU's heatsink is properly mounted. Athlons run fairly hot, and Poser 5 is practically a stress-test appplication.

When I put together my new Athlon system, I had the heatsink improperly mounted, but didn't realize because it seemed to run fine. Out of all my software, only Poser 5 put sufficient load on the processor to trigger a shutdown (my motherboard powers down automatically during heat-related crises, rather than locking up or burning out).