Window mode?

Hi, my crossfire game does not go full screen when I go in game even though it's using max resolution.

https://scontent-sjc2-1.xx.fbcdn.net/hphotos-xpf1/v/t35.0-12/11750966_956421754404691_456192696_o.jpg?oh=25e28ecf7f92cd2e9371e575f29b90db&oe=55AE5375

As you can see there are borders. How can I remove the borders without going to full screen mode, because it's annoying when I alt-tab it doesn't go back to native resoltion.

I am using windows 8.1