there is a couple of ways to solve this problem. I assume u have a widescreen monitor. I had the exact same problem as you, i wanted to play on 800x600 but my CS was streched and it wasnt in 4:3 aspect ratio like i wanted it
1. try going into nvidia control panel and find the option that lets u choose ur apect ratio (change flat panel scaling)
2. If that doesnt work you could download this program and make a custom resolution
http://download.cnet.com/PowerStrip/3000-2193_4-14972.html?tag=mncol3. This fixed my problem : I installed windows 7 and when i change my CS resolution to any 4:3 ratio resolution i have black sides on the
sides of my monitor.