I'm going to assume that you're using a widescreen monitor and are trying to play a game that as a non-widescreen aspect ratio. If this is the case, you have three options:
1. Find the graphics settings that you're looking for (would need more information about your system but it's usually a part of your graphic's control panel) and change the scaling from "letterboxed" to "stretched". This will cause the image to expand to fill the gaps but it will be appear awkward to look at. All this does is stretch the image like an elastic.
2. Change the game settings to adopt the wider aspect ratio. Older games often only utilize resolutions in the 5:4 (1280x1024) or 4:3 (1600x1200) aspect ratio whereas newer monitors have aspect ratios that are typically 16:9 (720p, 1080p) or 16:10 (1920x1200, 2560x1600). Some bitmapped games such as Diablo 2 are fixed resolution and cannot be changed. Games that are rendered using vector graphics have adjustable resolutions but may require workarounds. Check out http://widescreengamingforum.com/
for more information on your specific game
3. Do nothing