Patch 1.31 is a patch , not a full game. You must first install version 1.00 or 1.27 from your original CDs. Install both Reign of Chaos and The Frozen Throne .
If you have an older version installed (e.g., 1.27 or 1.28), you can sometimes find standalone 1.31 patch files to "patch up" manually without using the modern launcher. Technical Tips Warcraft 3 Frozen Throne Patch 1.31 Download High Quality