I don't really know when this started happening, but it just suddenly started for no apparent reason X.X
Basically whenever I play a pretty big game (eg: F.E.A.R., Sims 2, etc.) after about 5 minutes or so, it will stall. After doing so, there is no way possible to get back to the desktop, even the keyboard doesn't work when I press the Caps Lock key (The little green light doesn't come on) so I am forced to shut off my computer by switch.
I like to think I know computers fairly well, though I admit I'm no expert. I have seen similar problems in the past, but I never did actually successfully repair them. They went away when I got my next computer

:
My last experience with severe instability was with my old Pentium 3, about 6 years ago. Whenever I ran Diablo 2, my computer just spontaneously restarted itself after 5 - 10 minutes of playing (not shut off. Just reboot)! When I got my new computer (same hard drive, so it wasn't a software problem), the problem went away. I attributed the problem to either an unstable power supply or an unstable stick of RAM on the old computer. That computer was Really cheaply-built, grrr :mad
To this day, I'm not sure whether either diagnosis is correct, but my point is, the problem could *possibly* be a part of your computer is wearing out, and heavy applications may be forcing some component in your computer to overwork itself. Another option: Format your hard drive and see if it helps. Remember to back up all unreplaceable data (like music, saved games, artwork, wallpapers, any projects you have going, etc) before you do that.
It's reading F.E.A.R. as 50.28 MB or something, when it really is 5 gigs
A number of my programs do that, too. I'm not exactly sure why, but Windows just doesn't report the size of some programs accurately. When you actually do uninstall it though, it removes the entire application.
A few questions:
1) Do you use any programs other than games, that use a lot of computing power?
(Converting audio or video files, compiling programming code, graphics editing in Big programs like Photoshop, etc etc)
If so, are they equally unstable? For example, if you convert a full-length movie into another format, does your computer become less stable?
2)Does your computer have a Power light on the case? If so, when you do something that requires a fair amount of computing power (like playing or loading a game, or encoding a sound file) does the power light flicker at all? If so, a problem could possibly be a weak or unstable power supply. Power supplies DO wear out over time, as I've found out. I've had one die on me, and my Grandparents have had one die on them.
My computer does this light-flickering thing when I'm doing anything intensive, and I know I need to replace my power supply (my 300 W P/S isn't *really* big enough for a Pentium IV 3.0 GHz, 3 hard drives, a CD drive, and a Radeon 9550

: )
3) Does your computer seem to load or work slower than it used to?
4) Do you have any anti-adware programs, like
http://www.lavasoft.com Ad-Aware?
5) Have you defragmented your computer recently?
(I know, I know, I'm going into "housekeeping" procedures a bit

. I'm just going through a little checklist to try to figure out why your game would be crashing so badly.)
If I think of anything else, I'll get back to you, but I'm out of thoughts for now.