Hi! As the game offers no widescreen support, how could I play it letterbox? I mean, simply add to black bars to the screen and stop the stretching?

I think it depend on your monitor, some LCD monitor like Samsung have built in feature that can add letterbox on both side of the screen for game that didn't support the widescreen aspect ratio.

If not, you can try changing your video card setting. I don't know about AMD / ATi. But for Nvidia go to NVIDIA Control Panel (right click on desktop -> NVIDIA Control Panel), under "Display"
select "Adjust Desktop Size and Position", under "Scaling" try experimenting between "aspect ratio" or "no scaling" mode. Apply the change. It will retain the game original aspect ratio without stretching the image.

As wormholewizards said, try looking for an option to change your monitor from 16:9 to 4:3.

I was searching around the internet for hours looking for a way to do this but I never managed to get it working while keeping a 16:9 resolution. Luckily my LG monitor has an option to adjust the aspect ratio, so I just set the windows resolution and the game resolution to 1024x768 and then changed the aspect ratio of my monitor.

It worked like a charm.

Thanks, guys!