NHF – new heroes for Heroes V 3.1.

the thing is, I tried downloading and installing patches, and it says the game isn't installed.

I remember, I also couldn't patch the game. It turned out that the problem was that I had reinstalled Windows, and the records about the game installation and its path were, of course, deleted. If you don't want to reinstall the game, you need to add these records to the registry. I don't remember exactly how to do it, but you can easily find information about it online.