ARMA 3 fails to start (drops out of fullscreen)

[ARMA 3 fails to start (drops out of fullscreen)]

Tried running my ARMA 3 today, but it won't start up. Whenever I try all I see is the game flickering back and forth between fullscreen and Windows, then fails and hangs in Windows. After that I have to kill the task.

I tried renaming my profiles folder to be sure it wasn't a configuration problem. Didn't help a bit. I have no clue how to fix this, and I can't change driver as I'm using the new nvidia GTX 770 which only supports one driver version (320.08).

Using Windows 8 64-bit.

[2 Steam accounts with ARMA 2 CO + PMC/BAF on sane PC problems]

Hello.

I have 2 Steam accounts, both with ARMA 2 + ARMA 2 CO + BAF + PMC DLC, so I can have two DayZ characters.

Problem is that everytime I switch between my Steam user, both the BAF and PMC DLC stop working. When I start up the game I get this error: "Failed to load file: D:\Games\Steam\steamapps\common\arma 2 operation arrowhead\pmc\addons\air_pmc.pbo - decryption of headers failed". So if I go into Steam and select properties for both the DLC titles, run the 'Verify Integrity of Game Cache' I get the option to uninstall BAF/PMC. After doing that and starting up Combined Operation once again, both DLC will again install and function... Until I switch Steam account... again that is!

And before anyone asks.. Yes I do run everything as administrator (UAC is disabled).

Anyone got a working fix for this?