It sounds like a bad power supply. If you purchased the PC retail, it probably came with a cheapo generic underpowered unit that will never reach rated wattage. As you add pc powered devices, it overloads it and causes the pc to reboot.
I had this problem myself, and unless you have alot of added on devices, a good PSU (power supply unit) rated between 400-500 watts should work. I personally recommend Antec, but there are several other good PSU makers.