You don't happen to have a NVidia 6600GT by any chance? My piece of crap XFX 6600GT did exactly that, on any DirectX9.0c game.

Anyway, random crashes when you're not doing anything in particular sounds like an overheating graphics card, a screwed up driver for it or your sound card, or faulty RAM.

You should run memtest overnight to check that your RAM is fine. Free, downloadable program.

Install the latest drivers for your sound card. Also experiment with graphics drivers, particularly the latest but also a couple of older ones. Remember to clean out the driver completely before downgrading, this is really important with NVidia's drivers. There's a free tool you can find on the net for it, can't remember its name but I think it was DriverCleaner or something like that. Try the Omega drivers as well, some report them to be better than the official ones.