This is actually a graphics setting although display devices may also provide this feature. I don't know what graphics card & associated tools Mr. Dias has or if he is still even interested in other solutions two years later, but I think this information is of use to others with this issue:
The NVIDIA Control Panel for my GTX 460 SE provides three options for scaling: full screen (which will stretch the display in either axis if necessary), aspect ratio (which will fill the screen on at least one axis and letterbox the other if the selected resolution doesn't match your display's aspect ratio), and no scaling (which will do exactly what you want, which is to use letterboxing in both axes if necessary, so as to not upscale the image at all). See also this screenshot and the How can I force letterboxing on my display? question.
My old Acer Aspire 3620 laptop ran Windows XP and the software for its Intel onboard graphics also provided the option to letterbox on both axes for lower resolutions, though I wouldn't be surprised if there are graphics settings tools that do not.