PC crash on render

When I try to render my scene in blender using cycles, it gets to about 4-5 samples and crashes. When I say “crashes”, I mean my whole computer just shuts off. I am running windows 7 64 bit with a core 2 duo and an Nvidia 9800gt. I am rendering using the CPU because GPU rendering doesn’t work on my system. Does anyone have any idea why my computer is crashing?

if you dont use the graka-gpu mode for the cycles render, than
everything is done from the cpu - you even can create a render
without grafik-output (render via commandline - for examples check the relevant forum-parts).
You should check with other software and if its using same parts, it might
“shutdown” your computer too.
A first suspect would be to check the power-supply. Its only 5 month ago,
when i had a bad power-supply with those bulging, “brewing” capacytors
inside. Those problems starts slowly and happen first with software using
the hardware to the limit. Mainboards have often a menu to watch the
power-voltages on the different lines (±5V, 12V … 3V… ) and if those
values “sway up and down” its probably a faulty power-supply.
Other points are bad cooling, that might shutdown the computer if its getting too hot.