Windows 7: BSOD while playing games, ntoskrnl.exe

Windows 7 Professional 64bit, source: MSDN Academic Alliance.
Hardware bought during last week of March 2011, system installed at that time. Everything ran flawlessly until around 2 weeks ago.
Computer acts normally until I launch a game (tested with several titles: Dead Space 2, Guild Wars, Forsaken World, Sword 2). Then, within around 5 minutes, usually one of three things happens:
1)everything freezes, like if I pressed a pause, reboot required
2)freeze like above, but screen turns into a storm of colorful pixels, reboot required
3)BSOD, system saving some data and then auto-reboot
Other that that, everything is fine. No random reboots, graphic artifacts, sudden performance drops.
It might be without meaning, but I noticed that either freeze/BSOD happens withing first few minutes after launching a game, or doesn't happen at all (in that case, everything is OK even for several hours, until next reboot).
What I've done so far: updated video driver (mostly cuz of point 2), ran 2 night-long mem tests (one for regular RAM, second for video card memory), monitored hardware temperatures, did some general system clean-up, like registry clearing, disk defragmentation etc. Everything seems to be in order.
As you may notice while reading contents of zip file, perfmon seemed to be unable to recognize my antivirus - it's Avast Free Antivirus.
Any help or suggestions will be appreciated, as I'm currently out of ideas other than full system reinstall, or starting replacing parts of hardware.

Easy around two weeks ago you installed Avast


I notice your avast which is often at least a contributing cause of BSOD'S . Please remove and replace your Avast with Microsoft Security Essentials AT LEAST TO TEST


http://files.avast.com/files/eng/aswclear5.exe

Microsoft Security Essentials - Free Antivirus for Windows

I've been using Avast since beginning, but I guess it could get some troublesome program update. Anyway, worth trying, thx for advice. :)

I have no way of knowing when you installed it (my apologies) but it was modified on 11/28/2011

At first it seemed that removing Avast fixed the problem. System ran smoothly whole weekend, despite the fact that I tried to recreate situation, in which freeze occurs, like a dozen times. Then today, when I started to believe that problem was solved, system froze 3 times in a row. (Murphy's law? )
But thx for help so far. :) If anyone has any other suggestions, I'll be grateful. :)