I have a 7-Zip file on my (Windows 7) desktop with the extension .7z.
I was mistakenly led to believe that Windows 7 could handle it like it handles its own "Compressed (zipped) folders".
So I did the whole "Open with..." thing and got the extension associated with "explorer.exe".
Then when I double-clicked on the file I just got the wait/hourglass cursor flashing like crazy. But that's not the problem... the problem is that I now want to un-associate .7z with explorer.exe, or at least associate it with something else, but the "Always use the selected program to open this kind of file" checkbox is disabled.
Apart from the specifics of the mess I created for myself, the general question is, if the "Always use the selected program to open this kind of file" checkbox is disabled, how does one enable it?