There should be a program folder called “startup” in there.
All files listed in that folder is also executed on system startup.
Check to see if the notepad file is in there?
If so, remove it and that should solve your problem.
I've followed your advise and removed notepad file from startup. It did not appear on the next reboot. But how about the original cause? How to fix it? Or is it just an one-time problem and notepad message was just an annoying leftover from it?