What are we talking about? We're talking about playing a non-widescreen game, on a widescreen monitor, without it stretching. For example: Sid Meier's Pirates! does not support widescreen. It will support a resolution of 1600x1200. How do you play it on a 1920x1200, at a 1600x1200 resolution, without stretching? The same thing goes for Warcraft III, or any other game that doesn't support widescreen.
What you're looking for is "Aspect Scaling." This tutorial can't cover every particular option found in every driver set, or monitor firmware, but what you are looking for is an option that mentions "Aspect Scaling."
Scaling within the monitor is often your failsafe. It can produce the desired effect, if your video card does not. Many monitors offer three scaling options: Full Scaling (stretch, zoom, fill or some such), Aspect Scaling, and 1:1
Video card options are found the "Panel Properties" (inside the advanced portion of the display settings). Rather than step through the different mouse clicks, screenshots are included. This is a somewhat moving target, as control panel layouts can change with driver versions (particularly between card generations).