|To decode an .EXE you need a dissembler. A dissembler will only decode to assembler code and unless you understand assembler code it wont make a lot of sense.|
What you are never going to do is decode back to the original source code. That's impossible.
Notepad ++ is nothing but a text editor for creating source code that is then complied into an .EXE. You need something a lot more sophisticated to reverse the process.