.nfo files are just .txt files with a .nfo extension. Just right-click the .nfo and choose Open With. Select Notepad from the list and check the box that says "Always open with this application". Now whenever you open a .nfo it will open in Notepad.
Optionally set your Notepad font to "Terminal" (or any fixed width font) and the more extravagant .nfo files will be rendered correctly.