Mod might want to move this to hardware/software hell at this point.
Get a good copy of that (have another computer handy? the steam comp is suspect, but might be able to do it if you get lucky (and have a burner on it, etc etc)
Boot to it, and let it run for awhile. (It's completely automated unless you stop it and adjust things.) A while meaning overnight, if you can. If it finds errors, you have problems with your memory. (or god forbid, the memory controller on your cpu, but that's unlikely. Possible, but unlikely)
If it finds errors... remove all but one memory stick, reboot, let it run til you're satisfied that stick is clean. Remove and replace with next stick. Keep them seperated (little pieces of tape with a, b, c, d are helpful)
Keep going til you find the bad stick. Then keep going and test the remaining to make sure you don't have more than one bad stick.
If no bad sticks found, you can look elsewhere, (hard drive, eek!), but start with memory. It's free to test, barring the cost of the download and the price of an optical media)
Rule #1 of hardware diagnostics (well actually rule #1 is backup everything you can't afford to lose, so we'll call this 1.5): Test the memory first. It's free to test, and if you get back a clean test, you don't even have to crack the case.