In your NVIDIA Control Panel or AMD Software , set scaling to "Maintain Aspect Ratio". This adds black bars to the sides but keeps the image from stretching.
The Heroes of Might and Magic 5 community has been quite active in creating patches and fixes for the game. A community-created widescreen fix can be applied to the game. This usually involves downloading a patch that adjusts the game's rendering to support wider screen resolutions. heroes 5 widescreen fix best