On Windows 10 PC:

1. 
Download latest emacs from 
https://www.gnu.org/software/emacs/download.html Windows

2. 
Unzip it at C:/Users/UserName/Downloads/

3.
Change properties of .tex or .txt file so that double-click will open it with
C:/Users/UserName/Downloads/emacs-xxxxx/bin/runemacs.exe

4.
Find and download hunspell-1.3.2-3-w32-bin.zip from
https://sourceforge.net/projects/ezwinports/files/

5.
Unzip hunspell-1.3.2-3-w32-bin.zip
at C:/Users/UserName/Downloads/

6.
Add the following to C:\Users\UserName\AppData\Roaming\.emacs
(add-to-list 'exec-path "C:/Users/UserName/Downloads/hunspell-1.3.2-3-w32-bin/bin/")
(setq ispell-program-name "hunspell")
(setq ispell-really-hunspell t)

Note that you can move emacs and hunspell to other place than Downloads, and change corresponding settings.