Case "Always use the program..." grayed out forever?

Hello,

First of all I want to say that I’ve already spent hours searching before posting, I’m giving you a recap of the proposed solutions and why they don’t work.
If the topic is miscategorized please tell me but I think this is the most appropriate place! Thanks to those who will take the time to respond.

Here I opened a PDF file by right-clicking, open with, choose the default program, and there I wanted to try to open my pdf with something other than FoxitReader (a harmless-looking operation no doubt...), I accidentally chose Outlook (who knows why) and the box “Always use the selected program to open this kind of file” was checked, I hadn’t noticed.
Now, it seems grayed out forever, even though it’s unchecked, I can no longer choose my default program (so my little FoxitReader) to open my PDFs!
Of course opening a pdf with Outlook doesn’t make much sense, but now all my PDFs have the Outlook icon and I can’t open them any other way than “right-click + open with + my reader”.
Initially it was fine but now it’s very cumbersome to do every time, that’s why I started unsuccessful searches. Here they are:

screenshot of the problem: (edit I can’t because you have to be a member, oops)

- Logical, normal solution: use the computer’s built-in functions for this: http://windows.microsoft.com/fr-fr/windows/change-default-programs#1TC=windows-7
(doesn’t work because the box is still grayed out)

- Solution by changing the “keys” (I’m not a nut huh I just know how to follow tutorials): by running regedit.exe then changing stuff, something like:
http://www.zebulon.fr/astuces/187-desactiver-le-service-d-association-de-fichiers.html
http://support.microsoft.com/kb/555076/fr
(problem because I never have the same subfiles as in the tutorials)

- Disabling the “UAC” did nothing

- Then I’m a bit afraid to try angry stuff like: I’ll delete Outlook and then the computer will have to find a program to open my PDFs B-)

How to ungray this box, open my PDFs with the program I want? And by the way why does this happen, is it a deliberate Microsoft move?

If you need screenshots to help me better please tell me; I’ll sign up!

Thanks

Configuration: Windows 7 / Firefox 35.0