Right click on CSS in your Steam Library -> Properties -> Set Launch Options... -> In the box put "-norborder" without the quotations.
Now launch your game, go to your video options and put the game into Windowed mode. Restart your game.
BAM, full screen mode without the border around the window so now it looks like it's in full screen but you can alt-tab easily.
I also suggest you put "-novid" in the same box as -noborder, it will stop the beginning movie from playing. It should look like this: "-noborder -novid"