Title says most of it. My copy is from Steam and has only the Galactic Conquest DLC. Is there perhaps a bug from alt-tabing to much?
I had this problem too, at least, I think I did. I had all the DLC sans the Nomads as well as the Campaign, and the game only showed the main menu background, and crashed to the desktop whenever I clicked. It eventually fixed itself after I redownloaded it a few times, so try that.
Okay I will try that thanks 
Is there also a way I could get it to not boot in windowed mode?
Click on Options, in the bottom-right is the fullscreen checkbox.
The menu wont appear on boot-up therefore there is no options menu to use. Is there a way I can get to to boot in fullscreen before actually launching it?
Sorry, that was a bit of a fail on my part. If you open up “My Documents/My Games/GratuitousSpaceBattles/prefs.ini” then you can change windowed to 0.