Computer shuts down -- Advice?

Hey I've searched around for the answer to my question but most of the answers I've found are not applicable. Either it's a really old computer or a broken fan or some such item is the source of their problems.

Before I go out and spend money to replace things, I wanted to ask to get some second opinions. I'll hold off my opinion till the end so I don't persuade you in anyway.

First,: The SPECS:

CPU: AMD Antlon 64 X2 dual core
PSU: 650w
GPU: GeForce 8800 GT
4 gigs ram.

HISTORY:
Bought a stock HP pavilion Media Center m8000n pc about 3 years ago. Immeditately after purchase I replaced these parts:

PSU: I can't remember who makes it but it's a 650w power supply that I purchased for around $100.

GPU: Nvidia GeForce 8800 GT

I also bought 2 more gigs of ram.

The computer ran for 2 1/2 years. I brought it out to college with me and it worked perfectly.

THE PROBLEM:
This summer I went home; leaving the computer at college. When I returned, I booted up my computer and decided to play some CoD 4 with my roommate after we had gotten the place settled. We figured it'd be the only time we can play in a while. After about 15 minutes my computer went blank, no more noise, no more lights. The computer had never done this before.

I booted it back up thinking something weird might have happened, I thought maybe it was like an old car that hadn't been run in a while. Sometimes it takes a while to start and it'll shut down for a little while but after use it fixes and runs like a charm. I logged back into vista and tried to play again. It shut down after about 5.

I thought: "ok, maybe it's too hot." I'd had heating problems before. So I opened up the case and stuck a fan next to it. I also monitored the heat. Not a heating problem.

It shut down. I cleaned the computer out thoroughly hoping this would fix the problem. Maybe it was clogged and over heating. (all of these happened kind of with each other so I hadn't decided it wasn't a heating problem before this)

I'll skip a lot of steps I've taken and give the readers digest:

Computer shuts down without warning:
- When moving files (generally over a gig)
- When playing CoD4 (after about 2-30 minutes) (this one makes me think overheating)
- As soon as I plug in a firewire harddrive (which has worked in the past. It's how I brought all my files home with me)
- After a while of idling
- Trying to watch a movie (after about an hour)

Steps I've treid:
-Clean it
-Blow fan into case
-Reinstall OS: Vista --> Windows 7 (both had the problem which reminds me:
Important information I forgot

The reason I had to install windows 7 on the comtpuer is because when I tried to do the built in restore from Drive D it would always fail. I would complete the installation and then it would say it needed to restart, it would restart and normally it should boot into the screen where I install windows for the first time, but instead it couldn't load that screen and would shut down after a while​

- After that I installed UBUNTU just so I could download windows 7 and that install worked flawlessly
- Then windows 7 installed fine.

My Thoughts:
I'm somewhat computer literate but I don't know the details of how they work. I'm wondering if it could be a bad motherboard that is having trouble passing the information from place to place or compiling the information from everything. I.E when playing CoD4 it has to work with the graphics, the RAM, and Harddrive. The inability to even take a firewire makes me think it's the mother board.

I also wonder where HP keeps it's restore drive I can't seem to get rid of it no matter how hard I try. Is that stored somewhere off of the hard drive?

Any advice would be great and I apologize for the lack of organized thought.

Also if you need anymore information, or if I forgot to mention something just point it out and I'll give ya what you need.

Thanks a ton for any help!

 

check the event viewer for any errors that hopefully got recorded before the shutdown.
richt click Mycomputer>Manage>Event Viewer

 

Critical 8/28/2009 6:29:03 AM Kernel-Power 41 (63)

It just says that the system rebooted without cleanly shutting down first. This error could be caused if the system stopped responding, crashed, or lost power unexpectedly (this is what it feels like. Basically it acts like it would if the power went out only we don't loose power)

 

check the psu with a multimeter or swap it out with a spare if ya got one.. could be a faulty power supply from sitting. If the pc was left plugged in while you were gone, there could've been a power spike lightning or other factors at play there. In such a case it wouldnt matter if the pc was on or off, it can still be affected if just left plugged in.

 

It wasn't left plugged in. I stored it well. I'll see if I can get my hands on a spare PSU. Maybe swap it out of a friends PC. Also what's a multimeter?

 

a multimeter is a tool that reads the power oputput of an electrical lead.