I have the same problem with 2 ASUS laptop with optimus technology and using the Nvidia 540M

Setting the game to use the Nvidia Card only helps out a little bit but still the game crashes rather frequently. I notice that updating to the latest nvidia drivers has helped a lot, however the crashes continue.

As many of you I initially thought this issue was related to the optimus technology, however installing the game on my desktop with 8gigs of RAM and a Nvidia GTX 480 1.5gb Ram Overclocked, the problem occurs, although less frequently the game still crashes in window mode even without switching to other windows. In fullscreen all you have to do is alt+tab and is instacrash.

Another machine with lower specs running an ATI 4870 card with 512mb of memory, does not crash at all.

Which makes me believe this is only occuring with Nvidia Cards, anyone having the DirectX problem or similar with ATI cards?