(getfiled) and (startapp).

Here is a handy little routine that displays a dialogue box that defaults to a user defined directory and allows you to choose a Lisp file to edit. It then opens the file in Window's Notepad. When you have finished editing, it automatically loads the relevant Lisp file.
It uses the (getfiled) function to locate and open the Lisp file, and the (startapp) function to run Notepad.exe. You can change both path specifications to suit your setup.

(defun c:test ( / fl)
	(setq fl 
	   (getfiled "Select a Lisp File"
		     "d:/cadken/cadlisp14/NewLisp/"	
		     "lsp" 0)
	);setq
	(startapp "c:/windows/notepad.exe" fl)		
	(load fl)
	(alert (strcat "\n " fl " Loaded"))
   (princ)
);defun
(princ)