Ok so try this:
Start > Control Panel > nVidia Control Panel > Set PhysX configuration > In the drop down, select your graphics card; DON'T leave it on auto. Give that a bash and report back. Mine used to lock up, and then you'd get "Windows recovered from a driver fault" and it'd return to normal. I changed that to GTS450 from auto, and it hasn't skipped a beat in months. Also there were some new nVidia drivers released yesterday. May as well go ahead and upgrade those before you chance the setting as it'll set it back to auto.