I'm having the exact same problem as the OP. I've spent the past day or so trying to figure this out, because initially I thought it was a hardware issue. Now, I'm becomming convinced its a software issue of some sort (either DG or drivers etc).
I am running an 8800GT card with a E8400 Core Duo Processor, 4GM RAM. My issue is exactly the same-- within a minute or two of starting a new game, the screen goes black, sound stops, but the computer keeps running as usual (fans spinning normally, power running cleanly, all lights on, etc). I cannot alt tab/ctrl alt del out. It requires a hard reboot to get back to normal, if I let it sit the fans will just merrily run without any change. Windows event viewer is not showing anything occuring. I have unchecked the relevant settings in vista to try and get a blue screen with no luck (i.e. unchecked the auto-restart button etc).
Steps I've already tried:
1) Cleaning/Reseating the video card
2) Checked temps on everything (CPU running at normal temps the whole time, GPU heats up but only to ~60C which is normal for the card)
3) Ran windows basic memory test
4) Unplugged all but the primary harddrive and ran it (to check the Power Supply, which is a high end 650W one).
5) I'm using the newest drivers, does anyone have suggestions of which old nvidia drivers may be worth trying?
All the components are within 1.5 years old, so I'm pretty sure its not a hardware problem (unless of course the card is just inexplicably dying). I only experience such errors with one other game (Left 4 Dead, which used to run fine) but it is a noted problem with the NVidia drivers/8800GT card that they still haven't fixed. Any thoughts?
EDIT: I too have Realtek onboard audio, and just updated the drivers (will try and see if that fixes the problem).