Desktop PC shuts down randomly

About two weeks ago my desktop pc just started shutting down randomly for no apparent reason. It gives no warning messages at all it just seems to loose power and the screen goes black and the pc is entirely off. I was lost so I took it into a local shop they had it for nearly a week trying to find the problem and they finally called and said it was overheating and they just took it apart and cleaned it. Well I got it back and it stayed on a lot longer this time so I thought it was fixed, next thing I know it goes off. What are some suggestions? anything i can run to see whats causing it? I downloaded two different temperature readers that tell cpu, harddrive,acpi(??) and they are all very cool, under 40c...cpu's never over 37c.

specs-- P4 2.66ghz, 350watt psu, 768mbram, 120gb hd, geforce4 graphics card. thanks

After starting Your PC for 5 minutes, open the cabinet and check whether the Cooling Fan is working or not!!!

Caution: Open the Cabinet carefully and if not sure ask somebody who has done it earlier

any other ideas? i took this computer to a different computer service store and they've had it for a week still scratching their heads. I've called the tech twice, (b/c they never call me), he says it's narrowed down to the graphics card and the motherboard....if it's the motherboard I'll just buy a new computer maybe off of newegg for $500 b/c i need a reliable desktop. this laptop I'm using is quite slow (256mb ram)

If they have narrowed the case to Motherboard, then Go for a new Desktop, But a Branded one(HP,Compaq,Dell)

Hello,

There could be a possibility your computer is infected with malware. Please post a HijackThis log in the malware removal forum for inspection and report back here when Belahzur says your log is clean.

okay i'll try that when I get my pc back.

maybe the PSU is going south

it turned out to be the PSU, this is the 3rd one I've had with this computer, the computer tech replaced it with a higher quality one this time though and installed a case fan to keep it cooler so hopefully it'll last longer this time. thanks for the help.