Do you have "Graphics Options" or "Graphics Properties" when you right-click on the desktop?
If you have "Graphics Options", choose "Graphics Options" -> "Panel Fit" -> "Maintain Aspect Ratio".
"Graphics Properties" will open up a new menu, and you'll have to look for things like "General Settings" and "Scaling". Select "Maintain Aspect Ratio".
"Center Image" will display game at its native resolution in the center of your monitor. Unused bits on the edges will be your black bars.
"Scale full screen" will stretch your game screen to the full edges of both the X and Y. Might be ugly.
"Maintain aspect ration" will stretch the game image UNTIL it hits an x or y limit. So, unless the game supports widescreen, it will likely stretch to the top and bottom of your monitor, and not quite to the horizontal edges.