How to create a default association for files with no extension

To create a default association for files with no extension, open Folder Options > File Types > New Type and apply the following settings:

Description of type: Text (you can type in any description)
Associated extension: . (just a period)
Actions: New
Action: Open
Application used to perform action: C:\Program Files\<pnotepad_folder>\pnotepad.exe

Wether or not a specific filetype is added if non is given is implemented by the program. This is not a default Windows setting.

Since I don't know what you are using it for I can't really recommend any editor, but I have been using notepad++ for years now and it has most, if not all, features you expect from a decent text-editor. See http://notepad-plus.sourceforge.net/ for the details.