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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.