I'd like to make this post short, but there is too much to this story that can't be omitted, so here goes:

I bought the parts for a new PC from newegg about a month ago.

The config goes basically like this:
Case = Antec P182
Mobo = Gigabyte p35-DS3L
CPU = Intel E8400 (3ghz)
RAM = 2x1GB g.skill ddr2/ 2x2gb corsair enthusiast brand
GPU = 8800GT
PSU = Enermax Modu82 525W
DVD-r = Liteon generic
HDDs = 1 640gb WD 7200rpm; 1 300gb WD Velociraptor

Everything was installed and I opted for Vista Ultimate x86. The operating system was...not paid for in full, let's just say. Anyway, I began to get these random episodes of the entire system shutting off and restarting. This would occur only while I was gaming. The puzzling part, however, was that I only play Warcraft III, or occasionally WoW on the PC, neither of which is particularly demanding on hardware by current standards. I did a fair bit of research on the issue but unfortunately, the best replies I located all said "either PSU or Mobo problem." Without a reliable diagnosis, however, I was not convinced that I had to buy a replacement for either part.

I got the notion that perhaps the OS was at fault. In response, I purchased from newegg Vista x64 (long story behind choice of 64-bit, but mostly I had 6gb of ram and I wanted to plug in the 2x2gb pair on top of the 2x1gb pair that I had been using with Vista x86) OEM disc. The os arrived and I installed it clean. Everything seemed to be working alright for a week or so, then the random restarts resumed. I was baffled. These continued for a few days, at which point I randomly chose to run the BIOS updater for the gigabyte board. This appeared to have fixed the issue, as it didn't happen for a week again. Then the restarts resumed

So, being convinced that the problem was some sort of x64 compatibility issue with a piece of my hardware, I decided to try the old trusty xp that I used on my last computer. Well, XP installed correctly, but after I installed several hardware drivers, I was mired in the following loop: OS requires restart to complete installation of X driver --> restart proceeds to Windows XP logo startup screen (with the scrolling blue bar) then PC shuts off and restarts --> screen with "Windows could not load last time: would you like to 1) start normally; or 2) use last known good configuration; or 3) safe mode" appears. (menu items ordered by relevance, not by actual order).

So, if I try "start normally," I repeated the steps prior to that screen and just return to the same place. If I enter "last known good configuration," the OS loads without ANY drivers or programs as though it was the first bootup If I load into safemode, the OS is somewhat functional but the event viewer reveals no culprit.

Vexed by this loop, I happened to install WD Data Lifeguard utility while in safemode to see if I could locate any potential problem with the HDs. The first question the program prompts the user with says "support for >137gb HDs is not fully enabled, would you like to?" Of course I click yes, and to enact that registry change, XP restarted. Well, on reboot, everything worked well --hallelujah! So I am thinking "home free!" Maybe somehow the OS wasn't properly interacting with my Velociraptor drive, although the reasons for that were not clear to me since my XP install disc contained SP2, which I thought had >137gb support. I didn't care though, I just rejoiced at functionality.

Then, this morning, the loop returned. I tried to boot up, and the system failed, followed by a restart and prompt to start normally. I tried start normally three times, whereupon the system booted into XP normally with success. At this moment I am just leaving the PC on continuously so I don't lose access to the OS. (thank god for my second drive, without which my files and stuff would be long gone). Does this instability sound familiar to anyone?

I think I have ruled out heat as the culprit, here. I installed a thermalright ultra 120 extreme for the cpu, and an arctic cooling accelero passive for the GPU, to which I attached a nexus fan. I also have the center HD cage removed from the 182 with a nexus fan in its place. My airflow is designed to intake in the front and exhausted in the rear/top. I have followed airflow suggestions from this forum, and speedfan shows my system temps (in both operating systems, for consistency's sake) to be around 40C maximum with the CPU reaching about 44C at its height of utilization.

At this point, I am fairly convinced that I have some piece of hardware giving Microsoft Operating Systems the middle finger, but I don't have a clue as to how I might diagnose such an issue. I really do not want to deliver the PC to microcenter to pay $75 to a technician who will tell me that I need to buy all new parts. Can anyone help? Or tell me to whom I can speak to find help? I don't think that my OS should be this unstable, and it's never been in the past so I am pretty sure the install discs aren't corrupted. Any help that anyone could offer would be GREATLY appreciated (I'm willing to offer paypal compensation for finding an effective solution).