Everything Hypermood said is great. Windows does not just randomly cause rebooting. People want to blame the OS when 9 times out of ten it is hardware related or 3rd party software related, and can often be traced to SOS - silly operator syndrome.
Maybe post your hardware specs (mobo and CPU, cooling, etc) and get some of the OC'ers to comment. One simple thing to check is the CPU max safe temperature in your BIOS. If that is set to a low threshold, then as soon as your OC'd CPU gets a little hot (as it would doing some very intense graphics rendering in Halo back when you maxed the video settings) then it will reboot the computer for protection. Try setting the threshold temp a bit higher, then investigate whether you need better cooling for your OC'd CPU.
Comments
Maybe post your hardware specs (mobo and CPU, cooling, etc) and get some of the OC'ers to comment. One simple thing to check is the CPU max safe temperature in your BIOS. If that is set to a low threshold, then as soon as your OC'd CPU gets a little hot (as it would doing some very intense graphics rendering in Halo back when you maxed the video settings) then it will reboot the computer for protection. Try setting the threshold temp a bit higher, then investigate whether you need better cooling for your OC'd CPU.
Dexter...
here are my specs
AthlonXP 1600+ (when oced its oced to a 2200+)
Abit KX7-333 (non r)
256mb PC2100 (soon to be 512)