Computer help forum: Computer randomly shuts down

Hey my problem is my system keeps shutting down, during idle or nominal use. At the time it was happening I didn't have a Temp program running so I don't know what the Temp was at shutdown. I changed Surge Protector, PSU Power Cord, checked all internal connections. I've made no recent Hardware or Software Changes. Scanned for malware and viruses found none. I air dusted my system.

The PSU is about 4 or 5 months old, Currently this is the longest it's gone with out shutting down, about 20 min. It was to the point where not being used for a 24 hour period once windows started it would shut down, This also happened while I was in the BIOS settings to see if anything has changed. So it's not just a windows thing from what I can tell.

If anyone has any other clue as to what I can check or look at let me know.

I have a 450W PSU, 5 case fans, 1 CPU Fan.
My average CPU Temp ranges from 52C-56C During Idle.
Case Temp is around 32C-33C

Windows: Windows Vista Business (build 6000)
Memory (RAM): 2047 MB
CPU Info: Intel(R) Pentium(R) 4 CPU 3.00GHz
CPU Speed: 2995.8 MHz
Sound card: Speakers (Realtek AC'97 Audio)
Display Adapters: VisionTek RADEON X1950 Pro XGE | VisionTek RADEON X1950 Pro XGE | RDPDD Chained DD | RDP Encoder Mirror Driver
Monitors: 1
Screen Resolution: 1280 X 1024 - 32 bit
Network: Network Present
Network Adapters: Marvell Yukon 88E8001/8003/8010 PCI Gigabit Ethernet Controller | VMware Virtual Ethernet Adapter for VMnet1 | VMware Virtual Ethernet Adapter for VMnet8
CD / DVD Drives: D: LITE-ON DVD SOHD-167T | E: LITE-ON DVDRW SOHW-1213S | H: SX6426B EVM308A
COM Ports: COM1 | COM2
LPT Ports: LPT1
Mouse: 5 Button Wheel Mouse Present
Hard Disks: C: 111.8GB | F: 232.9GB | G: 74.5GB
Hard Disks - Free: C: 43.2GB | F: 132.6GB | G: 66.6GB
USB Controllers: 5 host controllers.
Firewire (1394): 1 host controllers.
Manufacturer: American Megatrends Inc.
Product Make: Custom
AC Power Status: OnLine
BIOS Info: American Megatrends Inc. 1010.001 09/07/2005
Motherboard: ASUSTeK Computer Inc. P4P800-E