System hangs when idle

I often leave my computer on overnight, etc, and i also use it to ICS for my Xbox 360, so it is idle a lot. Every now and then, my computer just hangs, and when i try to move the mouse, it just stays still, and nothing will change from the last frame. Nothing works, so i just have to restart it. When this happens, my xbox live connecvtion is disconnected, and the event viewer shows an unexpected shutdown, so i know that it has stopped doing anything. The system is in my system<<<
My question is simple, although i fear the answer is not, and i have no idea what other details to give: Does anyone know what could cause this problem and does anyone know how i could fix it.
If any more details are needed, just ask, and i will respond promptly.

Check power supply voltages are within 5&#37; (ideally with a multimeter), check for swollen/bulging/leaking capacitors on the motherboard, check the fan on your graphics card is spinning and it's not hot to the touch, check all your PCI cards and cables are securely connected.