Please post your Direct X Diag (Google if you don't know how) and RGL_log (found in your main Mount & Blade: Warband folder)
Could you also try to install the non-steam version of the game if you haven't already:
Warband:
www.flyingsquirrelent.eu/mb_warband_setup_1152.exe
Napoleonic Wars:
www.flyingsquirrelent.eu/mb_warband_napoleonic_wars_setup_1003.exe
Then download this zipfile, and extract the contents in your main warband folder:
http://dl.dropbox.com/u/11240073/wse_crash_log.zip
Run the game with wse_loader.exe, once your warband crashes, it should create a file "wse_crash_log.txt" in your main warband folder.
Please post the contents of the file.