Go find a .cs file, and right click it.
Select "Open With..." in the menu.
In the dialog that pops up, pick the "Select the program from a list" option.
In the resulting list of programs, find notepad, select it, and make sure that the "always use this program to open this kind of file" box is checked.
Hit OK, Notepad will now open all .cs files when you double click them.