After about 3 minutes of being outside in the starting area, the game will crash in one of two ways. Both ways my screen will freeze but the game will continue running; for instance I can hear my footsteps, fire my gun, etc. Now for the differences. If I just run around outside for approximately 3 minutes the screen will freeze and when I press escape the game will crash with the usual Windows crash message.

Sometimes after this has happened Direct3d will stop functioning. DxDiag will report that no D3D device is present and other games will give an error message stating that the Direct3d renderer could not be found or initialized. A GPU monitoring program called GPU-z gave me the impression that the video card had turned off or something as all of the sensors were blank.

The other way it crashes is if after about 3 minutes (but before the screen has frozen) I open any containers or bring up the pipboy the screen will freeze, pressing escape will trigger the CTD and my video driver will crash (at least I think that's what is happening). I'll briefly end up on the desktop in a very low resolution, possibly 640x480, and 8-bit colors. The screen will flash and then my monitor will go into sleep mode and wont wake up, forcing me to restart the computer.

This is the only game that causes this problem for me. Other more graphically demanding games (crysis 1 and 2, assassins creed 2, fear 3) run better than fine even on my old hardware. In addition Fallout 3 runs just fine on my computer.

I should also mention that I have tried lowering all settings as low as they go and raised them as high as they go and this doesn't change the stability of the game at all, the time frame for crashing remains the same.

The innards:
Asus A8n VM CSM
AMD Athlon X2 3800+ 2.0Ghz (2x1.0ghz cores)
Nvidia Geforce 9800 GTX+ 512mb (drivers up to date)
4x512mb Corsair RAM
Corsair TX 650w Power supply
Windows XP 32-bit SP3
Up to date directx 9.0c

Possible "problem child" is my sound card which is older than old but has never given me any issues and provides some excellent sound quality and options. Its the SoundBlaster LIVE! Audio c800. They haven't provided updates for it in a long time but they did release a kind compatibility driver package so it would work properly on newer systems.

Edit: Forgot to mention that when I checked the Windows crash log the crash was listed as a c0000005 access violation in FalloutNV.exe.

I have exactly the same problem, any savegame, or new game, crashes to desktop and requires a reboot to restore graphics to normal. The strange thing is, the 'change user' hokey on my keyboard will bring up the usual prompt in Windows, in normal colour depth and resolution. My system specs are as below

Asus Striker Extreme
Intel Q6600 Quad 2.4Mhz
NVidia-Asus GTX460 TOP 1024MB Gfx (Latest drivers
4x Corsair Dominator PC6400 Ram
Enermax 760W Power
WinXP SP 2 32-bit
Creative X-Fi Fatal1ty Audio

All up to date. This fault only started when I updated the game recently with the Dead Money and Honest Hearts DLC this weekend (I'm a svcker for bargains). I hadn't played the game for approx 3-4 months previously, so didn't go thru all the patches that 'may' have been added in that time.