Need Help Identifying Possible PSU or Power Suply Unit Problem

I'm having a problem with a computer that I built 2-3 1/2 years ago everything worked fine up until 4/20/2013 when it just started shutting down instantly for no apparent reason. It first served as a server with Windows Home Server 2011 for a half a year. Then served as a FreeNAS server for three weeks before it was put in storage for 2 weeks. Then served as a backup desktop computer in my office after a motherboard exploded in flames of glory and now it runs for a while then shuts down for no reason.

PC Specs:

Motherboard: Gigabyte GA-G41M-ES2L

Micro Processor/CPU: 2.80 GHz

RAM: 2 GB

CD/DVD Optical Media: (MASTER) 1 (DVD/CD Read/Write) and (SLAVE) 1 (Read DVD/CD Drive)

Floppy Drive: 1 (Half Inch Floppy)

HDD: 2 - 500GB SATA HDD

PCI: 1 (Wireless Network Adapter) and 1 (Ethernet Card)

Power Supply: Allied ATX 300W Switching Power Supply (Model #: SL-8320BTX)

Any help would be appreciated that's helpful.

 

When A computer is turning off for no reason, It can be only a couple of things....
1. Virus
2.Heat getting to hot/ bad cpu fan
3. or Power supply going bad/ out...

 

Well, we can eliminate 1 thing. It's not a Virus because it's running Ubuntu 12.10 64-bit edition.

 

Check all fans, especially over the CPU... Next check or replace Power supply with a Brand Name one..

 

it actually was the CPU fan that was loose. thanks for your assistance.