I changed some configs in the Config app and now the game doesn't stretch when I change the resolution to full screen.
If I'm not mistaken it did stretch in the original config, I alredy reset the configs to the default value but still my game shows up in a little square in the middle of the screen.
Check your graphic driver settings utility, you need to look for the "image scaling" setting.
On AMD/ATI cards this is in the Catalyst Control Centre,
under "My Digital Flat Panels" ->
check "Enable GPU Scaling", and then you have to change your desktop resolution to something below the max to be able to set the scaling options, because AMD are dipsticks.
"Maintain Aspect Ratio" is the better option here as it won't make everyone look massively fat and short if your on wide-screen, but you can set it to fill the screen if that is more important to you.
Apply the scaling setting, then change your resolution back.
It will quite likely show a different setting in the greyed out boxes to the one you chose, it does this, don't worry it hasn't forgotten, it's just lying to you.
That's it, done.
Sorry I can't give directions for NVIDIA, I don't know what their drivers look like any more, you should be looking for the same phrases though.