Nevermind, I managed to solve the problem eventually.
Just in case anyone else has this issue, I solved it by:
-Right clicking 'Alien Swarm' on steam and going on 'Properties'
-Then 'Set Launch Option'
-Typed '-sw' (without the ' ') so it starts in windowed mode
-Then from there, I changed the display option to my resolution through the game menu.
I know for many this would of been the obvious thing to do, but I'll mention it anyway.