Rar is an archive file format, and as such contains compressed data. So you'll have to extract its contents in order to open the compressed files in Notepad. Here's a freebie that does the job: http://7-zip.software.informer.com/
Strictly speaking, there's no such thing as exclusively notepad-type files: the .txt format (the one associated with Notepad by default) is quite common, and these files can be opened with a lot of different programs. In addition, Notepad isn't quite picky in terms of what kind of files you're trying to view with it: gibberish or not, it will show you practically anything (make sure you're not trying to feed anything heavier than 500 Kbytes or so, though). So in order to view a file with Notepad, any file, you can simply right-click the file, select 'Open with...', and pick Notepad from the list.
Don't forget to untick the 'Always open' checkbox if you do decide to do that.