Yep - it sounds like your Desktop Screen Resolution is (well - was) set to one size (say 1024 x 768) and you play POTCO at another size (say 800 x 600) and when you exited the game the Direct X control messed up on close - causing windows to lock in at the lower setting.
Follow Mir's steps above to see the slide bar and increase it a notch, apply the change, and see if it looks better...
