64 bit shuts down randomly

I built my rig two weeks ago and it ran fine without any problems. But from last three days when i keep my PC on at night for downloading it shuts down unexpectedly. When i woke up in the morning i found my system was off. Processor temperatures remain around 40 C. I have nvidia 196.21 drivers. Computer shuts down normally because when i started it again it doesn't showed that windows has shutted down unexpectedly(at the time of boot up). So its definately not the power supply problem.

Windows 7 is build 7600

My rig is,

i7-920
asus p6t deluxe v2
Corsair XMS3 3 x 2 gb DDR3 1600 Mhz
Nvidia GTS 250
Cooler master Hyper N620


System works fine when i work on it, but the problem occurs when it is idle.