I would right-click on the file, choose "Open With" and then select Notepad from the list -- that should take care of it.
To avoid having to do this every time, choose "always use this app" on that same screen. That will set your computer to always use Notepad (not Internet Explorer) when you double-click the file. Of course you can always go back to the "open with" option if you want to use something different in the future.
