Windows – How to stop Notepad++ taking back file associations

I've been using Notepad++ in Windows for some time, and while I think it's a good editor, I decided to go back to Emacs to try to learn to make the most of it (writing some elisp and such).

As I do like Notepad++ I don't want to uninstall it, but I'm failing to get notepad++ to stop taking back file associations for certain file types like .txt and .xml.

If I right-click on the file, choose "open with > Choose default program…" and set it to the emacs windows client, I can see the icon flicker shortly, as if it changes, but then immediately changes back again.

I've disabled all file associations from the Notepad++ settings, so the "Registered Exts:" list is completely empty.

Does anyone know if there is something else I can do to stop it taking back the file associations?

(just thinking: is it possible that I need to reboot after disabling the associations?)

EDIT:

Thank you all for your help.
Unfortunately, I haven't had much time to use at figuring out what's causing this. So I installed Notepad++. Strangely enough, this wasn't what was doing it, and when I now try to associate it with Emacs, it does the same flickering thing, and next time I do a normal "Open" it asks me what to open the file with (so, while it's not bound to Notepad++ any more, it still doesn't allow me to rebind it to Emacs. As a test I've done the same with the normal MS Notepad, and it also doesn't allow me to rebind to that… so, it seems it's neither a Notepad++ or Emacs issue, but something else (Windows itself?) stopping me rebinding the files.

I just tried going through Control Panel > Default Programs > Set Associations and it currently says:

Name            Description                           Current Default
.txt            TXT File                              Unknown application

I've tried changing it from here (through Change program...) and that doesn't change anything. It still says "Unknown application" and it still opens with the "Open with" window.

As a side note, should I rename my question, now that it seems to be unrelated to Notepad++?

EDIT 2:

I just tried changing the association of file that was never associated with Notepad++ (.css associated with Notepad2), and that has the same (lack of) effect. Just to cement that this is definitely not a Notepad++ related issue.