Assuming you are working on Windows and talking about the Notepad program commonly distributed with it, you can invoke the program via the OS using system or exec, depending on whether you want to pause script execution or not.
The difference between system() and exec() is NOT whether it pauses the script or not! system() will pause the script (unless used in one of a few special ways), exec() will start the notepad AND NEVER RETURN!
Jenda Enoch was right!
Enjoy the last years of Rome.