Problem with Notepad++ Portable v 5.1.1

hi all,
i have a problem with my notepad++ portable application.
i installed it in a directory and i wanted to adjust my notepad++ default notepad application.
so i made some adjustments.
while doing this adjustments, when i changed Notepad++Portable.exe file's name to Notepad++.exe and double clicked on it i got an error which says 'Another instance of Notepad++ is already running. Please close other instances of Notepad++ before launching Notepad++ Portable.'
i controlled and there wasn't any other instances of Notepad++.
but the strange thing is, i didn't get any errors when it name was Notepad++Portable.exe.
when it name was Notepad++Portable.exe and when i double clicked on it, it works normally.