Thanks for the quick and detailed response. Couple things I'd like you to try, I'll list them in order of ease.
You could firstly try extending your virtual memory. Looks like you're not getting runtime errors any more (tell me if this isn't the case) right ? Might be experiencing the sudden shutdown problem now. Loveless has brilliantly covered instructions on how to do this in
this post.
Update your XP to the most recent service pack.
Update your 5200 to the latest drivers available
here.
Are you running multiple monitors? Do you have a high (higher than 1024x768) screen resolution? If yes to either, disable the second monitor or bring down your resolution and try playing Fiesta.
Could you also provide details for your motherboard? A long shot suspicion of mine is that it might be a conflict between your onboard video card and your 5200.
Also when did Fiesta start playing up on you? Did you install any software before it happened? Was it after a recent patch or does it do this on a regular basis? A system restore might be in order. Restore your system back to a time when Fiesta was working.