Fun fact: on modern Windows, if you uninstall modern notepad using the Settings app to get the old one, you won't be able to associate .txt files with notepad.exe without a registry trick: https://superuser.com/questions/1750222/how-to-open-file-typ...
Modern Windows has big problems with file associasions: default to web browser, sometimes asks, sometimes it chooses a random program (for images). Old Windows (9x, 2k, XP, 7) was better.