Computer restarts

[Ninjutsu PiloT]

I have a gaming system which I've had unchanged for over a year. I have recently noticed that when I play video games, the computer restarts, in fact during high CPU loads in general it is prone to restarts and rarely, but sometimes it restarts when idle.

I have tried software program after software program for scanning for viruses, running registry checks, video card checks, memory checks and can't find anything. I have checked event viewer and noticed some patterns and have disabled all of those programs. the computer restarts less frequently (more so with games) now that i've done that but no more events come up after a restart.

I formatted my computer and re-installed nothing but windows xp SP1, Call of Duty 4 and AIM and I still get the restarts. I upgraded to SP2, same thing. I now have auto restarts on Guild Wars and Vanguard Saga of Heroes.

I changed my 680 PSU out for a 480 (only spare PSU I have to test with) and I couldn't get the computer started. I ran a power requirement check on my computer using some website and it says it requires 400 something watts base so I wasn't surprised switching the PSU had no effect.

Anyway here we are, still unable to play any games on my gaming PC. Is there anything anyone else can suggest before I blow 100 bucks on a PSU that may not work anyway?

Here is my computer at a glance
ASUS M2N32-SLI Deluxe Wireless Edition AM2 NVIDIA nForce 590 SLI MCP ATX AMD Motherboard (http://www.newegg.com/Product/Product.asp?Item=N82E16813131011)
AMD Athlon 64 X2 5400+ Windsor 2.8GHz Socket AM2 (no link available)
OCZ GameXStream OCZ700GXSSLI ATX12V 700W Power Supply 100 - 240 V (http://www.newegg.com/Product/Product.asp?Item=N82E16817341002)
OCZ Gold 1GB 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) Desktop Memory (http://www.newegg.com/Product/Product.asp?Item=N82E16820227123) X2
EVGA 320-P2-N815-AR GeForce 8800GTS Superclocked 320MB 320-bit GDDR3 PCI Express x16 HDCP Ready SLI Supported Video Card (no link available)
Western Digital Caviar SE16 WD5000AAKS 500GB 7200 RPM 16MB Cache SATA 3.0Gb/s Hard Drive (http://www.newegg.com/Product/Product.asp?Item=N82E16822136073) X2

If there is anything else that I can post to help diagnose this problem I'll be happy to do so, I check the forums often.

EDIT: Drivers are up to date
EDIT: Sometimes my computer doesn't restart but the video just freezes and a weird sound can be heard over my speakers, a beeping kind of sound, the kind you get when you put a cell phone near speakers
EDIT: Temperatures are normal, used multiple temperature gauge software including the mobo utility program
EDIT: While I was troubleshooting this problem I noticed it got very quiet, I looked in my case (panel off) and saw the heat sink fan had turned off, which has never happened. the cpu was about 50C when it did it

[slytherock]

I blame the PSU

[calumc]

get something to monitor your temperatures

[Ninjutsu PiloT]

I'll edit the above post but temperatures are normal, I have a few programs watching them for my system, CPU, and GPU.

[Ichbin]

Have you tried taking everything apart and putting it back in. Thats helped me sometimes.

[Greco101]

Could be early signs of a bad mobo. That happened to me when my capacitors blew on my old PC and it eventually got to the point where it would only stay on for 20 seconds... sometimes I'd get lucky and it'd stay on for up to 20 minutes! It really had a mind of its own.

Maybe take a look at your mobo and look for blown/leaking caps. If you don't see any, don't write it off yet. If possible, try and find a known working mobo that you can swap all your hardware to and see whatcha get.

EDIT: Just for fun... my old motherboard (http://i16.photobucket.com/albums/b5/BlackS3ven/ouch1.jpg?t=1197259443) :oP