Computer permanently freezes, requiring hard reboot

Okay this is kinda getting out of hand. Whenever my computer freezes, everything stops, except the fans AFAIK. Mouse doesn't move, sound stops, the whole works. This comes with no warning, and it can happen at any time, although it's much more likely to happen when my computer is running under load.

Things that may cause the freeze:
-- Playing Dominion on LoL and be on Mumble (happens consistently)
-- Running tons of flash applications, such as youtube videos, streams, and flash games from Newgrounds
-- Rendering a very long video

I've considered the possibility that something was overheating, but the symptoms of overheating are either not there, or pretty unnoticeable (occasional frame skipping in games). The worst that has happened from a couple gaming marathons was frame dropping from 120+ to 30 or so, but ironically the freezing didn't happen.

What's the fastest way to determine the root cause of the problem?

Play with an open case (don't worry PC's run at 12 volts only), and check your videocard core temperature on the topside of the card (which you see when it's installed). Just tap is a few times with your finger to see how hot it gets, sure it will get warm, this is normal, but if it starts to get painfull (+60 degrees celcius), there might be something wrong.

Also, check your PC case (also the videocard) for dust which gathers around / inside coolers.

Also you can try an overclock tool to DOWN-clock your videocard core AND memory (to about 80% it's default setting) and check if the problem still occurs.

Freezes are most of the time related to overheated or malfunctioning memory (video or system ram), but when it happens when playing games, it can also be caused by an overheated GPU on the videocard.

When you say it freezes, does the last image remain on screen or does it go black?

Sounds like your problem is graphics related. It's quite possible that your GPU is dying. I wouldn't physically touch the components, usually you can just use various software programs to monitor component temperatures.
The absolute fastest/sure-fire way to determine the problem is to start swapping components and see if that solves the problem. If you can, borrow a compatible graphics card and swap to see if it improves things (make sure to install the required drivers for it).

I've had a very similar problem with my old computer. Turns out the heat dissipator (those metal things glued on top of the chips) was not having good contact with the video card's chip.
The computer would just suddenly freeze during games or otherwise heavy video usage. There was no frame rate drop.

Since it was still on warranty I just had the video card replaced, but I could have also bought a bit of thermal conducing paste to reestablish contact between the two parts.

What's the fastest way to determine the root cause of the problem?

For starters: http://www.memtest.org/

Still organizing my stuffs and prepping for the memtest >.< Hopefully I'll have more information later this week.

About the GPU overheating... that might be a possibility, but I just had my computer freeze with a 40 C GPU. No idea about the CPU, but the air wasn't blazing hot or anything. Also, does an overheating GPU really cause the sound to cut off instantly?

Still organizing my stuffs and prepping for the memtest >.< Hopefully I'll have more information later this week.

About the GPU overheating... that might be a possibility, but I just had my computer freeze with a 40 C GPU. No idea about the CPU, but the air wasn't blazing hot or anything. Also, does an overheating GPU really cause the sound to cut off instantly?

40 C for a GPU is nothing, those things can get up to 70 C or 80 C without any problem. So that 's not it. It can be the memory though. Both system RAM and video RAM.

But if nothing feels like overheating (you notice this quite quickly when you open the case and move your hand around), it's most likely malfunctioning memory. It's quite common for PC memory to stop working top notch after +/- 2 year of extensive use. Most people won't notice it since Windows 7 most of the time corrects these errors itself (old windows version showed the blue screen thingy when this happened).

The only way you can really test it is swap these parts with other ones and see if the problem is still there. Just remember to only swap one part at a time, not all at once.

But then again, it can also be another part of the PC that's malfunctioning (unstable PSU voltages under high load, wrong BIOS settings, etc), but again, freezes are most often related to memory issues.

And about the sound, if the PC freezes, so will the sound.