I think that you are talking about playing in non-widescreen resolutions (i.e. 1024x768) in games that don't support widescreen. In other words, if you want to play in a smaller resolution without the picture stretching out and looking blurry, you have to change the setting using your video driver control panel. In the NVidia control panel you go to Display > Adjust desktop size and position and then apply "no scaling" under the scaling tab. (You could also choose Aspect Ratio, but then your picture would not be stretched, but it might be blurry). I choose the GPU instead of the monitor to perform the scaling, but you might try it either way and see which way works for you.
If you are using an ATI card, I'm not sure how to adjust this, but I know there's a way to do it. You might want to search on the net for ATI scaling, aspect ratio, etc.
Anyway, I hope this helps...