Full screen problems

[yeeyoh]

Hi,
I've put off playing this game for a long time but I've been working through my games and I'm finally giving it a go.
I alt-tabed out of the game to adjust my volume and when I went back to the game it's in a windowed mode locked to the top left of my screen..
I was playing in 1600x900 which is full screen to me and 1600x900 is still what it says in the options. I've tried different resolutions and they seem to work but when I try to switch to 1600x900 goes full screen for a second and then seems to bounce back to this slightly smaller resolution, windowed and locked to the top left of the screen.
I'm about and hour into the game and it was working fine before, I didn't change anything but alt-tab out of the game.

Phenom II x4 955
16GB ram
HD4890 2GB
Soundblaster X-fi gamer OEM soundcard.
All up to date drivers and windows updates installed.
I tried looking through the forums but couldn't find anything. I might have missed it.
Anyway thanks ahead of time for any help,
Yeeyoh.