One might think it would be enough to replace the copy of Notepad.exe by a renamed Notepad++.exe. This may still work under Win9x/Me.
More recent versions of Windows are more careful about their untegrity, and they consider Notepad.exe to be part of it. As a result, they won't let you replace Notepad.exe easily.
Googling for "replace notepad" may bring you procedures corresponding to your exact OS version - the service pack level may make a difference.
Also, you may want to search the forums for that same information.
Do it and report fndings.