Hi Marxist, thanks for the reply.
I didn't change anything myself, some games open fine in the correct resolution or in a 16:9 resolution but a lot have the error.
I fixed the error when it occurred in 'Bioshock: infinite' by doing the following:
http://steamcommunity.com/app/8870/discussions/0/828937979229979741/
I also solved it in 'The Wolf Among Us' by pressing alt+enter to go into windowed mode and then change the resolution of the game when it was in windowed mode.
Unfortunately most games won't go into windowed mode and it's a massive pain in the arse to find all the stuff I need to edit in the system files.
EDIT: My GFX is Radeon HD6800 and Windows 7 64 bit.