There's several things I would try:
1) If you overclock your system...don't. UT is notorious for making overclocked CPU's that appear stable to lock up/reboot/implode.
2) Make sure you have the latest nVidia reference drivers, or if you're feeling lucky, you can try the latest Diamond drivers for your product. The new detonator drivers from nVidia are optimized for GeForce/GeForce2, and I believe there are older reference drivers that are better suited for the TNT. You can try
http://www.reactorcritical.com. Search their messagboard for "TNT" and look for a post in regards to "best drivers for TNT". That should give you some opinions on which ones to use if you're really looking for the best. My stepkids system is running the exact same card you have with the 5.xx drivers with no problems. If you do change the drivers...go to
http://www.tweak3d.com and follow the instructions in the GeForce tweak guide on properly removing/ reinstalling new video drivers.
3) Make sure you have the latest updates for Windows, as well as the rest of your hardware.
4) Try disabling other things, like sound, or running in software mode, to try and pinpoint the source of the problem.
5) You can always try reinstalling UT. That has helped many ppl who couldn't put their finger on the exact culprit.
6) See if you can identify when it happens. Does it always happen right after you first start, or usually a certain time period after it's been running? Freeze up offline as well as online? Try and locate a pattern and see if there are any discernable causes that relate to it.
Not sure if this will help...but it's a good starting point...good luck...
Jeff
