Serious Hardware Issues. Out of ideas. Please help

I'm at my wits end. I am having a very serious, and very hard to diagnose, problem. My PC has been randomly shutting off for no obvious reason. It has been happening for about two weeks now. Here are my specs.

asus a7v600 (onboard nic, onboard 5.1)
AMD Athlon XP 2100+
512mb PC2100
Celestica Radeon 9600xt 128mb
Thermaltake PS (can't remember what one exactly, butI believe it is 450watt)
2 Maxtor 7200spin Hard Drives (80gig, 60gig)
lite-on cd-rw 40x16x48
pioneer dvr-108 (installed the other day. Problems were occuring prior to this)
Windows XP Pro /w SP2

My problems started occurring when I upgraded to the newest Beta version Catalyst Drivers. I was running v4.7, and I upgraded to 8.04(or something close to that). My mistake was that I did not un-install the 4.7 drivers first. After updating I fired up Vice City, and lasted about 10 seconds into the gaem, and It froze up. after several seconds, I got a VPU error, asking me to reboot. Try again same thing, try another game same thing. I go to uninstall & it is missing some files and wont let me uninstall. Download ATI cat remover. Doesnt work. Reboot in safe mode. Doesnt work. After some messign around, I found all the registry settings that had anything to do with the ati driver, and removed them. Reboot, and install back to 4.7. Reboot, fire up Vice City. Same thing. Frustrated, I leave my computer. Come back few hours later to browse the internet. Pffft, monitor shuts off. Do a hard reboot, work for a few minutes not really doing anything demanding. Pffft, monitor shuts off. a few seconds later, computer reboots. Since then, this has been happening over & over again at random times. I cannot pinpoint it to anything that I am doing on my computer, or running on my computer. Doesnt seem to matter. sometimes it lasts only a few minutes, other times it lasts several hours (even over night and a good portion of the next day). It seems that it happens much faster when playing any type of games though. This is the only consistent thing. I am sure it is not a heat issue. I have Motherboard Monitor running, and I am average around 55-60 degrees c on my cpu 30-35 in my case. Never seen CPU go over 65. Also, the overdrive tab on my video card I have never seen go higher than 42. I have done a complete format of my system, and re-installed windows. Figured it was fixed, but a few hours later, it happened again. I updated my motherboard to the latest version, but it happened again. I tried playing COD while not connected to the net, and having the sound un-plugged (ya never know), but it happened again. I did a fully system virus scan, nothing was found. I am completely stumped about this. Tried everything I can think of. I'm pretty sure that it is hardware issue with my video card, power supply, or motherboard (or combination there-of). Seeing how it can sometimes be many hours before it happens, I can't really swap something out & determine if it works or not. I wont be able to get any hardware place to find the problem. I know they'll just fire it up, and run it for 5 minutes and see no problem, then just give it back, laugh at me, and take $40 bucks. What on earth can I do?!?