Here is the trick:
- Go to Preferences->Multi-instances->Default (mono-instance).
- Close notepad++
- Go to “c:\Program Files\Notepad++” and create there an empty file called “asNotepad.xml” (of course, you will need Admin privileges for that).
Result: When you double click a TXT file in Explorer, Notepad++ will open and show that new document in its own window/tab.