Spontaneous rebooting can be a heating problem. UT2003 is one of the more advance and complex games out there, so it could be overheating your CPU more than other games and apps.
If you're overclocking, lower the clock speed down to normal. Also you may want to inspect the inside of your PC for dust build up, especially on your CPU's heat-sink/fan. Disassemble the HSF, and clear the dust bunnies out, wipe the fan blades clean, and add a drop of lubricating oil in the fan bearings (under the sticker). Then reassemble it, and make sure you have some decent thermal compound (Arctic Silver, etc.) so the heat dissipates properly.