bcdedit /set increaseuserva 3000
If you use a 32 bit OS, your virtual addressing is limited to 2GB. Unfortunately Shogun 2 doesn't seem to be large address aware, and when it reaches the 2GB limit, can crash and cause other problems. If you have 4GB of RAM on a 32bit version of windows and have some / all of the problems I list below I recommend you try this fix...
Before this fix I had:
Repeated downgrading of graphics settings - anti-alias, texture details, shadows would all be downgraded even if I edited the preferences script to increase detected VRAM.
Unable to play in DX11 mode, game would crash within a turn on campaign map.
Unable to alt-tab game in DX11 mode, even on main menu.
Game would crash in campaign in DX9 mode after 30 minutes play.
Units would turn white after a few battles. All units textures in campaign and battles were white.
Crashes in legendary mode that corrupted my save-games leaving 0 byte files behind!
These are all gone now, I play in DX11 mode, I have extra graphics options enabled that I couldn't manage before (they would be auto downgraded). No crashes during extended play sessions.
To apply the fix READ THIS CAREFULLY:
http://www.twcenter.net/forums/showthread.php?t=238484
Depending on which OS you use, and how much installed memory you have, pick the option that best addresses the addressing problem.
2.5 GB - 3GB , no higher, you must leave space for your OS!!

I'm having problems with Shogun 2. It keeps on crashing during battles. When it crashes the screen will freeze and the audio will cut out. For a moment the audio will resume and the visuals continue for a few frames as well, but then Shogun 2 stops and returns to the desktop with no message at all. It is very odd and it has happened multiple times for a long time now. And I can't find a solution. Can any of you find anything wrong?
I believe there is a crash log as well (The .MDMP files?) but I'm not sure how to post tehm here.