A few possibilities:

Overheating
Malware (ie, virus)
Memory problem

Possible solutions:

Have you cleaned inside your computer lately? Use caution, and a wrist strap, when doing this to avoid damage. Make sure all the fans are clean, running, and not making any funny noises (Power Supply fan, CPU fan, and case fan).

Follow the recommendations and instructions in the links below (in my signature block) to help protect and cleanup your system.

Does your computer report the actual amount of RAM you have installed? Have you added any recently? Sometimes just removing RAM and reinstalling it helps.

I had the same problem too, ran adaware and spybot s&d and cleaned out temp internet files. that seemed to fix it, but I only used that PC for like a month after that, by then I had all my new parts for my new PC.

How long are you playin before the crash??
heat build up is bad, I know when I am playin UT on my new pc, it heats up my whole room, but I have 3 case fans running on it, plus I use intel monitor, for cpu and fan status, that way it alerts me of high temps.

I'm having the same issue on mine and I also have a Geforce4 MX 440 (not sure exact same) Heat may be the issue for me, or possibly a clean up inside my cpu.

I started similar thread with the link below. Wasn't sure if I should have new thread for new question but thanks.

http://daniweb.com/techtalkforums/thread35906.html

I have a Geforce FX5500 and I am experiancing the same problems with games like Jedi Night Academy and programs such as Ventrilo.

Could it be that my pc is lacking the watts to support the video card? I have a 250watt video card, 512ram Geforce FX5500 256mbs