Vista x64 randomly shuts off my computer!

First off: Vista sucks, I know. I shouldn't have gotten it.

Anyway, Vista's been shutting off my computer at random. Just completely, out of nowhere, it'll decide "okay I've had enough" and it'll shut itself down. No blue screen, no pause. It just shuts itself off without warning.

I've reseated the RAM and it passed the Vista memtest with flying colors, so I don't think it's that. To be honest, I'm clueless.

HALP! D=

Jam it back in, in the dark.

I have Avast installed, which was fine for me on XP (but then again... that's XP). I'll do a quick search around for Avast causing any problems like that, and run a scan (although the problem isn't rebooting, it's just outright powering down like somebody hit the button).

There's nowhere I can't reach.

Okay, I got SpeedFan installed a while back. It's reporting my temperatures hovering right now around 23-39 degrees C (depending on the sensor), but I've seen it climb up to around 50. I'll grab a can of air and give it a squeeze the next time it shuts itself off, and see if that does anything to help. Also, I can verify that all but one of the fans are working perfectly (and there are 4 total: one exhaust, two intake (the broken fan's an intake... I think, but either way atm I have an intake, an exhaust, and a CPU fan working), and one CPU fan).

I'll also take a closer look at the power button and the wires connecting it to the motherboard. When I first built the computer, it was really wonky and oftentimes I'd have to push it constantly for about 15-20 seconds to get it to power on.

Also, I am REALLY glad I got a new hard drive. I hooked the old one up to transfer all my furry porn files and documents to the new hard drive, and I could hear the old one grinding and clicking all through the transfer. I'll prolly send it in to get replaced once I figure this problem out and go all at having 900 GB of storage.

EDIT: Okay, it shut itself off and I powered it back on after a few seconds. Bios temps read the system at 53 C, CPU at 39 C. I also took a can of compressed air and cleaned out the inside of the case, getting rid of quite a bit of dust. Here's hoping.

This thing is sticky, and I don't like it. I don't appreciate it.

Don't worry, I've already taken care of that. Brand new hard drive with Vista on it. Also, one of my fans ISN'T working, but like I said, it's one fan not working out of three which ARE (including the CPU HSF).

Anyway, no change so far. Just got done playing WoW and it's holding steady at 48C tops. I'm starting to think it's the power switch, which REALLY sucks because I don't know if I can fix that.

I am a dolphin, do you want me on your body?

Fuck it. I took the computer into a repair shop. Either it's inexplicably overheating (despite the sensors recording it at 50C literally 3 seconds after it shuts itself off) or the power switch is fucked, which is something that I can't fix. Guy said it'll be ready by Tuesday, so I'll let you know then what the problem was.

I was speaking idiomatically.

Update: It's my PSU. Fucker's fan stopped working and it's been overheating (which is why the guy I took it to couldn't duplicate the problem I'd been having, since I keep my room fairly warm and his shop is much colder). He's replacing it now, and I'm out around a hundred dollars at best.

How ya doing, buddy?