Old Post but I know people will go nuts trying to figure this one out. Nothing here or from other sites worked for me. Some of the fixes seemed too general and I did not want to reset all, only fix the one. I'm an IT guy and tried the usual Windows Control Panel etc...
So... delete the UseChoice from hkcu. In my case I associated .shader with Visual Studio. After doing it, realized VS does not highlight code like Mono Developement, but couldn't switch it back.
Removing the key below re-enabled the check box. now I could re-associate with Mono.
For you, find the extension giving you grief in HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\
If it has a "UserChoice" key delete it. (Back it up if you are paranoid, right click export!)
I don't think I even rebooted and the check box was back.