If I alt-tab to desktop or another application at any point after having started the game, upon trying to switch back to it it'll crash giving a window saying "DX Error: Invalid call". Have reproduced many times, pressing ESC for the menu before alt-tabbing doesn't affect it.
Radeon X1950
Intel Q9550
Windows 7
dxdiag:
http://pastebin.com/VCzFwYuW
