Running FSX in Win7, after installing the latest 344.11 I started receiving "The procedure entry point Direct3DCreate9Ex could not be located in the dynamic link library d3d9.dll" errors when going from windowed to full screen or back. Would have to hit "ok" 9 or 10 times, then things seemed ok.
After some frustration and research, found a thread for an unrelated game that suggested removing the GeForce Experience, that did it for me. Seems it might be related to use of the ENB mod . . .
Just trying to save anyone else some grief . . .