does it always boot to the operating system before it restarts? or has it ever restarted during post also? the restarts are at random times correct? if you need assistance, ask someone knowledgeable to help you with the troubleshooting:
if it has restarted before the OS is loaded (even one time) than you have a hardware issue. you may have hardware misconfiguration (if this problem occured after you changed the hardware recently), a conflict, or failure. if it is randomly restarting while the OS is loaded, it still could be hardware problems, but it could more likely be from an operating system conflict or corruption.
the very first thing i would try is go to windows safe mode and see if you still get problems with the computer rebooting by itself.
than i would test the system out with another power supply, this is a very likely reason for those reboots. you may not be getting enough power to the motherboard or the power supply is erractic and starting to fail.
next i would try using the "fail-safe default" setting in CMOS setup. if it still doesn't work, try reseting the CMOS by taking out the motherboard batteries for 30-60 minutes and put the batteries back in (or just use the jumper to reset CMOS for 3 minutes while main power is physically disconnected).
if that doesn't work still AND you know that you recently changed the hardware configuration (jumpers, added a new cd-rom, ect), i would see if some hardware is conflicting through
windows device manager and see if it reports any conflicts. if none are reported i would still try to physically take out all expansion cards except what is absolutely required to run the computer. including motherboard, processor, hard disk, memory, and video card. than i would load fail-safe defaults in BIOS and boot to windows.
run windows until you either see the problem come back or for a long enough time for the problem to appear. if the problem persists, try using a different video card because some video cards do not work so well with some motherboards. and also try different memory because of that same problem. if you have no other parts to swap out than skip those steps.
at the last resort try reinstall your operating system. if you currently have win95, win98, or windows millenium try to reinstall with win2000 or winxp, if you have that option. take note after you install drivers if the system becomes unstable.
and if none of that fixes your computer i would assume hardware failure, there is not way to tell unless you can test each component out on another computer (processor, memory, video, sound,memory ect) and if all components work than it is logical to assume your motherboard is failing. if you can have someone test out the motherboard only, with their own parts than that would be good, but i doubt this is an option. taking it to a computer repair shop might cost as much as buying a new motherboard.
good luck troubleshooting togo!