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.