If you want to play Might and Magic in windowed mode, you will need to use a console command everytime you start the game up. The command is mat_setvideomode width height 1 (setting width and height to your monitor). However, we can automate this using an autoexec.cfg. Here are the steps to do this

1) Go to the folder where the game is installed and go into mm/cfg
2) Create a file called autoexec.cfg
3) Inside the file, add the line mat_setvideomode width height 1 (Change width and height to your monitor. For example, I use a 1440p monitor and the resolution I use is 2560 x 1400 so I use, mat_setvideomode 2560 1440 1. You can check your resolution from your window settings and choose according to that)
4) Once you add the line, save and close the file.
5) Right click the game in your steam library and under launch options add the command +exec autoexec.cfg and click ok
6) Click start and when the game finishes loading the main menu screen, it should switch to windowed mode automatically.

This was information gathered from forums and PCGamingwiki combined into this guide to make sure everyone can play on windowed mode. I have not played the game that much with this, but let me know if it bugs or causes any issues with this. It should essenitally just run the command mat_setvideomode width height 1 automatically so it should not cause any problems with the game.