Hal900x

Nvidia drivers used to have an option to force scaling to a fixed aspect ratio on the GPU, overriding the scaler in your widescreen monitor and letting you play your 4:3 games in their original aspect, with black bars on either side in fullscreen mode. This option was buggy as hell, rarely worked, and was finally removed in an acknowledgment of failure by Nvidia.

There is a registry patch here which will give you that option again. I use it even with a 4:3 LCD monitor because the game gets stretched a bit vertically without it.