Some games store your settings (like display resolution and keymaps) in an seperate file on your hardrive, e.g. GTA San Andreas saves the file gta_sa.set in the userfiles. If your game does this you can locate that settings file (google is your friend) and then delete it. The game will automatically recreate a new file when it starts next.
Alternatively, you can try the command line parameters on a link to the game exe. For that make a new link to the game exe and add the forementioned parameters to the target path of the link rather then the exe itself.
If both doesn't work it might be worth a try if the game supports windowed mode, start it up and press Alt+Enter to switch to windowed.
