I've config to use 3DFX. Gone into nglide settings and sure to set ratio to 4:3. I even set my desktop resolution to 1024x768 and graphics settings to maintain aspect ratio. Desktop looks correct 4:3, with black borders on the sides.

However launching Outlaws, it always stretches to full width screen again.

Can anyone help?

EDIT: Don't worry. Managed to sort it! Need to set the resolution of nglide to match my desktop settings.

In nGlide's config, set Screen resolution to "By desktop," then set Aspect ratio to "Preserve original." You should then be able to set your desktop resolution back to whatever you like.