You are referring to 1:1 bit mapping. Not all monitors supports this mode, but many do. It is found somewhere in the OSD menu, probably under Advanced Video options. I'm not sure 'cause I have a 4:3 LCD monitor at the moment. This means the monitor will only use the necessary pixels to display the game. For example, if you were to play Diablo II which only supports up to 800 x 600 resolution, then in the center of the screen will be Diablo II using exactly 800 horizontal rows and 600 vertical columns of pixels.
Another possible option in the OSD is Aspect Ratio which allows you to keep the game in 4:3 mode, but this will perform a "zoom" where the game will fill the entire screen with black borders on the left and right to maintain the aspect ratio.