Annoying Hitman: Codename 47 bug

[DarkRanger57]

Whenever selecting or firing weapons, a single mouse click won't fire or select the weapon. I need to double click every time i want to shoot. It's quite annoying.

It didn't do this in the tutorial, but started on the first real mission of the game. It really annoys me to the point where the game is more or less unplayable with this bug.

Any ideas on how I would go about fixing this, or am I pretty much screwed?