shutting down

adata ddr3 ram
700 watt power supply
CoolerMaster HAF 922 Gaming Case - Black
AMD Athlon™ II X2 260 Dual-Core CPU
ASUS M4A87T -- AMD 870
Liquid CPU Cooling System
24X Dual Format/Double Layer DVD±R/±RW + CD-R/RW Drive - Black
1 TB HARD DRIVE -- 16M Cache, 7200 RPM, 3.0Gb/s - Single Drive

If it shuts down over and over (and it's a shutdown/reboot, and not just power shutting off), it's likely that lsass.exe is being closed, which starts a 60 second system shutdown/reboot (on purpose) initiated by the NT AUTHORITY\SYSTEM account - because the local security subsystem is no longer running (lsass.exe), and the system's security is compromised. That is classic virus/malware behavior, particularly something sasser does (although there are others that could do this as well).