In all honesty, I am stumped, I have no idea which component of my PC is failing. But he symptoms are the following, will try to group in a chronological order:
First I started having minor hickups in different games. It was like the game was loading something, freezing the frame. The glitch was always followed by a sound like a fan stopped spinning for a second and started throttling up again to normal, or the HDD stopping and launching again. I took no heed and continued to play, ignoring these symptoms. I was playing Age of Conan, on high settings. Not highest, but still pushed my system, I guess. Also, a note that the loading times were humongous.
Then the freezes froze irreversibly. The sound of the fan \ hdd glitching was still there. The screen was littered with artifacts kinda like this - http://imageshack.us/photo/my-images/502/img0023h.jpg/ . The patterns differed each time, something the games were visible through them sometimes no, but usually yes. Sometimes the screen kinda chose the current average colour that was on screen at the moment, for example, in a game where there was a snowy location, the screen was a pure grey-blueish colour, and is froze like that. It wasn't an overlay or anything, just one solid colour.
So I was thinking the GPU was dying. I've got an Nvidia 440 GT.
Until my PC shut down and I couldn't turn it on until I reseated the powercable of the PSU. That happened once.
I changed my HDD, reinstalled OS, everything was clean. For a time it was without a crash or a freeze.
Until it all started again. With time the symptoms happened more often.
Today I was playing a game, I think it was Call of Duty: Modern Warfare 3 MP, it's free this weekend on steam. That game didn't crash often, like once every 3 hours. But I noticed my loadings becoming longer and longer. Decided to restart my PC. Did so, and everything I did in a game, any game was stuttering, the sound was glitching. Even in the menu the sound was effed up. The loadings because impossible to wait through, literally. Didn't manage to play anything. And this is where I am.
I am afraid I killed my PC through my constant ignorance of this, but I want to know, maybe I can get through with replacing one piece of hardware. I am not rich, sadly, can't afford a decent rig. D= If my specs are needed, I'll write them down. Also, I guess I could capture some footage of how the glitches sound/are.
Have you monitored temperatures while you were gaming? Maybe you can try something like HWMonitor.
You haven't given us your PSU info (brand, model #, etc.), but if it turns out the system isn't overheating, I would looks towards the PSU next.
Edit: I also forgot to suggest trying to run your computer with the side of the case open and use a normal tabletop fan to blow air into your system. If that helps mitigate the freezing and crashing, then it might be an overheating issue.
The temperatures while gaming stay fairly normal, and when I have these random freezes with the hearable hickup in the case there's no increase or decrease of temperature monitored. The peak was 51*C, and that wasn't when crashing or anything, just when launching a game, fell down to 46*C soon after. I'm guessing it's kinda normal.