Associate TXT file type with Notepad.

1. Click my computer, Tools, Folder options, File Types.
2. Click New.
3. Add TXT and click advanced.
4. Place Notepad in the Associate File Type drop down.
5. Click Ok.

If the user has Windows 10:
1.  Open Control Panel
2.  Select Programs > Default Programs 
3.  Select "Associate a file type or protocol with a program"
4.  In the list that appears, under the "Name" column, search for ".txt", double click on it, and make sure Notepad is associated.  
5.  Select "ok"