Well that's the issue. 1024x768 is not wide screen. It's an old CRT style 4:3 resolution.
If you can change that in game to something closer to your desktop resolution, then you'll have those black bars go away. The black bars are there to keep the aspect resolution at 4:3, where as your monitor is on something else (probably 16:9). that means that your monitor, when it "fills" to the top, still has space on the sides. So CS just blacks those out, since it has nothing to display there.
If you change the resolution, it'll change the graphics in the game to use the entire area, without looking distorted.