First of all, I'm probably going to make this a very long post so I can describe exactly what has happened, so if you take the time to read the whole thing, you have my utmost thanks!

So, the short story:

I built a PC in January. Gave it a mild overclock, did a little torture testing, everything seemed to be running fine. Installed Vista on it. Then, about a month later, I started getting random BSODs followed by an immediate reset (so fast you could never read what the BSOD said). This usually results in the system rebooting over and over again, never getting past the windows loading screen. I have to turn it completely off, and then on again, before it will boot. The frequency of the problem has increased over time, and every once in a while screws something up so bad that I have to reload my BIOS or reinstall windows just to get it to boot again. I've tried both XP and Vista, no difference. I've tried reducing the clock speed back to its default, as well as increasing the CPU voltage. This seems to alleviate the problem slightly, but its difficult to make that call because the lock-ups are so random. I really need some direction as to how I can begin troubleshooting this problem.

Now... the longer version:

I built a PC from the following parts.

Crucial Ballistix 2GB (2 x 1GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400)
OCZ GameXStream OCZ700GXSSLI ATX12V 700W Power Supply 100 - 240 V
ZEROtherm BTF90 92mm Silent UFO CPU Cooler
Intel Core 2 Duo E6600 Conroe 2.4GHz 4M shared L2 Cache LGA 775 65W Dual-Core Processor
GIGABYTE GA-P35-DS3R LGA 775 Intel P35 ATX Intel Motherboard
XFX PVT88PYDF4 GeForce 8800GT 512MB 256-bit GDDR3 PCI Express 2.0 x16 HDCP Ready SLI Supported Video Card

The SATA hard drive I ordered ended up being a dud. No OS would install on it, and it failed the WD Diagnostic tests. I bought a comparable WD one at Microcenter and everything was fine. After everything was up and running, I OCed the CPU to 3.0 ghz (333x9). The CPU temps reported by coretemp were always way too low (like 20 C idling), but I barely saw any difference in temperature when overclocking, so I wasn't worried. I left the CPU voltage at the BIOS default, 1.35 V, and was able to run Prime95 overnight, so I never increased the voltages. I also set my RAM to about 2.1 V (it's rated to 2.2 V), as well as the timings suggested by the manufacturer. About a month into everything running smoothly, I started getting the random bluescreens. When I was there to see it, the bluescreen would flash quickly, and then the system would restart. It would nearly always then get stuck in an infinite rebooting cycle. In order to get it to boot again, I would have to power the system down, and then turn it back on again. Every so often, something seemed to get corrupted, and it would stop booting altogether. I've had to reload the BIOS once, and reinstall my OS several times. There seems to be no difference between XP and Vista, although the problem began when I was running Vista.

Now, here's a few observations I've made that I believe can help someone diagnose the problem. While my CPU was still overclocked and I got a BSOD, the infinite rebooting cycle would barely get past powering on. The system would restart (lights come on, you can hear the fans going max speed) and then restart again (lights and fan go completely off, then back on) within a few seconds. It was almost like a car engine trying to turn over. After a few attempts, it would stay on for a while and then freeze somewhere around the motherboard splash screen. Powering the system down completely always solved the problem, although it would sometimes "turn over" a few more times before powering on for good. I've since reduced the CPU back to its default frequency, and this problem has disappeared. Now, when it gets stuck in the infinite rebooting cycle, it restarts during the windows loading screen. Once again, powering down completely solves the problem and allows it to boot normally. A few days ago, I increased the core voltage to 1.4 V. I believe that this may have reduced the frequency of the initial crashes, but it's still hard to gauge at this point.

Here's some more weird info. I can usually tell when the crashes are coming now. My computer will start behaving strangely. If I'm playing a game, the game might crash 2 or 3 times in an hour, with the last crash resulting in a BSOD and a restart. Sometimes I'll come home from work, having left the system on all day, and no programs will run. Each program will crash immediately when I try to load it, and give me some sort of error message (I'll try to post something more specific next time I see it happen). After about a minute of trying to get something to load, I'll get the BSOD and restart. Also, I've recently noticed that this sometimes messes up my video drivers. After I get my system booted again after a crash, the video will be extremely laggy (as if there were no drivers), and the Nvidia options are nowhere to be found. Another restart usually fixes this problem.

Whew... I guess thats all for now. I'm really looking for some advice on what I can do to determine the problem, since I've never dealt with an issue like this before, and I'm only an amateur at building PCs.

Thanks for any help, and let me know if you need more info!

That is a very interesting situation your compy's in. It could easily be any number of things causing your problems. First, try right clicking on My Computer and click on manage. Check your event viewer for any red errors that correspond to a crash, or lead up to a crash in your case (LAN errors also show up in red, so ignore those).

On the hardware side; run memtest, and see if you can cause a crash by pushing the CPU with your favorite burn in program. Try disconnecting all unnecessary hardware (extra add in cards, peripherals, everything that doesn't keep you from booting into windows). Basically carpet bomb for any weak link in your setup.

Thanks for the advice. I ran memtest while I was in class today. It completed 2 passes with no errors. I've also dropped the CPU voltage down to its default value in the BIOS (1.35v) to see if I can get more crashes . One odd thing I noticed. CPU-z is reporting the core voltage as about 1.312v when idling, and 1.296v when under full load. I know the drop is expected when under load, but the idle voltage is still about 0.04v under what it should be. Should I trust CPU-z over the BIOS? If so, could this 0.04v discrepancy be enough to cause problems?