First, my PC.

Asus A8R-MVP with a non overclocked 3500+
1GB of Corsair RAM in DC mode
AMD/ATI x1800xt (I've been playing more video games recently, so the second monitor is currently off)
Antec 450W SP PSU. (don't need the lecture, I already know about this.)
winXP w/SP2 (power options are always on, never set to turn anything off.)
Seagate 7200.10 250GB (main drive) and a WD AAKS 320GB data drive.

I woke up this morning and tried to use it, it rebooted. Thinking it odd, I went about my day as best I can, but it has rebooted 5 times so far. (been 100% stable all its life. The only problem I ever had is when the onboard NIC died, and I put an old 3com NIC in) Doesn't seem to matter what I do, it reboots at the oddest time. I thought it might be related to the FireFox update that happened last night, but even with FF turn off, I reboot while setting up the next map while playing TF2. PC probe reports all is well, the only rail lower then it should be is the 3.3 @3.23. Speedfan reports no SMART errors. Windows is NOT set to reboot upon error, it should bluescreen. There is nothing notable in the event viewer. CPUz reports everything stock, I've never overclocked this computer. PC probe reported case and CPU in the 30s, while ATItraytools reports the x1800XT at 55. (this is while surfing the net of course. It just locked up then rebooted on me again, FF actually saves as you type!) The machine just locks up, then 5 or so seconds latter, just reboot. Any clues?

EDIT: I just got done running hijack this and Adware, neither of which turned up anything weird. (I did remove whenusearch) I can run AVG, but I don't think the computer will remain stable long enough.

Well I have a few suggestions, but I am no expert so take my advise if you like.

1) Download Avast! Antivirus or any antivirus that can do a boot-time scan. Set up a boot time scan and see if any virus turns up. I like Avast! because it is free!

You can also try to run Kaspersky Internet Security. I know you said your computer is unstable but MAYBE it will make it through a scan and can tell you if you've got a virus/spyware.

2) This suggestion is far more brute force, but it solves a lot of problems. Format your computer and reinstall windows. Backup anything like documents, saved games, etc. and start fresh. I have had a few viruses that have done so much damage that it just was not worth trying to recover.

3) My last suggestion for now is to blame the PSU. I know you said your rail's are stable, but sometimes the rail is not the problem. If I were you I'd go to a retailer and buy a PSU. Try it and see if it fixes anything. If it does not then return the PSU and get your money back.

For the record I like Antec PSU's. I have an Antec TruePower Quattro 850 and it suits me just fine.

Good luck and hopefully one of my suggestions solves your problem. Post your results either positive or negative.

William

before you do any of that^^

take the computer apart, and get rid of all the dust in the heatsinks and fans, and reapply thermal paste. i know that pcprobe reports temps in the 30s, but i wouldnt trust em.

next run a scan disk.

then try the above.

Could be hard drive. Have you tried safe mode at all? If it runs better there, then maybe you can fix it from there.

I suggest working out if it is a hardware or a software issue.
1. Boot up your system using a Live Linux CD and see how stable things are running as much as you can.
2. Have you changed graphics drivers?
3. Does it do it with the case off (i.e. is it overheating?)
4. Remove any extra hardware and see if that helps (sound card fault or psu problem), take the DVD drive or a hard-drive off to reduce the power drain.
5. Swap the grpahics card if you can and see if that helps.

I am very special with my computers. I sit behind my router, windows if fully patched. The only thing adware found were tracking cookies. Hijackthis found the whenusearch thing. That was all that was on my computer. I will run AVG if I get the chance, but I doubt it will find anything. I will see if I can find a different PSU to use. The SP PSUs from Antec have a known capacitor problem. I hope to replace this PSU some day.

I didn't try safe mode. I did not log on after one of the reboots, and it didn't reboot at all. (although I had to unlock my computer this morning, so it didn't reboot all night.) My mouse pointer does turn into an hourglass right before it happens, so I wonder if this is a problem with a program/driver.

I will see if I can get a bootable DSL and try that. I haven't changed anything in my system, other then allowing FF to finally update itself before all this happened. My case cover is still off after plugging my brother in laws harddrive into my system. (500GB WD AAKS SATA) I just transfered some things to his computer, then gave him the drive back. I didn't feel like unplugging all the wires in the back so I could put the side back on. Looking at the temps, I'm not sure I need to.

Not really any extra hardware. The only things in there are the video card, and the PCI NIC. I use onboard sound. I'm the only person near me that I know of that uses PCIe, so swapping PSUs and video cards will be tough.

Edit: Forgot to mention, there is no dust in my machines. I clean them out at LEAST twice a year, sometimes more.

Can you remove your gpu and still run?

I suppose I could, but I wouldn't be able to see anything. I might be able to find an old PCI video card around here. I'm not sure that would rule out a video problem or PSU.

I grabbed my 8" clip fan and pointed it at my PSU. (I did this after the last time I rebooted) I have so far been 100% stable. I am starting to suspect my PSU...

I also grabbed memtest, I still need to burn it to CD, perhaps that will turn up something. (never had one before, does this sound like a memory issue to anyone?)

It could as most mem issues are random, quirky

just a little thought,seems like a hardware problem,,because you are not reporting any error messages,so the usual suspects are either the ram or the psu,can you switch your ram around and try them 1 at a time..??

But Im betting psu since your fan discovery

Memory errors should give you a blue screen and a memory location address of the faulting string.

It could be a fault in the Windows kernal but that should give you a blue screen and the faulting program code, file, dll, etc.

It wouldn't hurt getting a new powersupply. It's a long shot that it's the powersupply. But I've seen strange things. If it's not the powersupply it will be one component you don't have to worry about when upgrading in the future.

Have you tried running Windows error checking on your main hard drive.
To access the tool right click on the hard drive you want to scan. The tool is in the properties window.

Thats what I'm thinking to. The computer probably rebooted 10 times yesterday, but hasn't at all today after I put the fan on the PSU. If it stays up for the rest of today, I'll try turning the fan off to see if it starts up again.

Guess I should go to Antec's website to see how long the warranty is.

What's an Antec SP 450W powersupply.

Ten years ago I used Sparkle oem powersupplies. Then I couldn't get them anymore and switched to Antec powersupplies. The only Antec powersupply I had a problem with was a 350W that was bundled with a Sonata case. I replaced it with an Antec true power 380W. That was five years ago and the computer is still up and running with Vista without a hardware upgrade at a friend's home.

Current Antec Powersupply warranties are five years.

I would agree with meltwater another thing you might try is restoring to a few days ago and see if that doesn't solve the problem. How old is that PSU? PSUs tend to get less efficient as they age so that is something to consider. Also make sure the heat sinks inside the PSU are clean. Dirty PSUs can overheat and shut down as well as the entire system for that matter.