Win Server 2008 R2 automatic shutdown problem

Hey guys, I am new here. I am using Windows Server 2008 R2 X64, just recently(the day I post) the computer shut down automatically. I not sure what's the problem, there are no dust in my pc/PSU cause they're still new....and I do recent checkup once 3 mths.
Yesterday(15/4/10) don't have the problem at all ....untill today. The temperature reading are good, and normal.
CPU temperate between 35-40(maximum)C
GPU I used the ATi Overdrive to enable manual fan control to 100% w/ no OC any system, temperate reading at 50-56(maximum)C
CPU cooling is cooler master v8 at 100% speed, and casing is cooler master CM690
CPU fans all set to 100%, btw I am using cooler master pro-m 700W PSU....
this pc is with me for past 8 months, custom build pc by myself.
but why when I played W3FT : Xhero Siege map in bnet....twice it shut down by itself, background running just windows media player playing some songs...and with mozilla on....