I suggest grabbing a good anti-virus scanner and running a full, comprehensive, whole-day-if-necessary scan. I suspect a virus infection.
There have been no Windows Updates or game updates that should cause DEP to suddenly hate your game.
No. I just tried it. I did a full all day scan. Nothing. And I can't turn off DEP for the game. I'm just gonna re-install the game. Delete everything except the saves. If that works, I'll inform.