I downloaded Notepad2 and placed it in
C:\Program Files\Notepad2
I wanted to associate all .txt files to Notepad2. I did an Open With and Choose Default Program. I browsed to and selected Notepad2 and clicked OK. But, the application didn't show up as an option in the Open with dialog box.
I moved the Notepad2 directory to Program Files (x86) and tried again, same problem.