In short, Win'7 just doesn't like "Heroes 3"! :(
What to do:
1) The easier option: install the game on a PC with Win'XP and copy the folder with the installed game to a PC with Win'7;
2) The more complicated option: install Win'XP on the same PC, and then install Heroes on it.
Another important point: it's better if the installation path doesn't contain Russian letters or spaces (for example, D:\GAMES\HEROES3); Heroes might install, but installing WoG on them is guaranteed to cause problems. Although you can rename the folder beforehand.