I've been having a pain getting this fixed...
I dont really know how it happened, I suspect it was related to CCleaner, but can't be sure. I went to mount an .iso file and got an error saying something to the like of "No application has been setup for this file type" - While thats not the 100% exact way it was said, that is what it mean.
Now, I also noticed that the icon had changed, and instead of being Daemon Tools Lite icon, it was another icon, akin to the windows "I can't find your icon" icon.
Now, at first I couldn't even go "Open With", and thus I restored the backup CCleaner made, as I had just ran it for the first time since my laptop's HDD had died (about a month or two back)
This let me set the "Open with" to Daemon Tools. But, now I get from Daemon Tools "Error in Command Line". Looking around on my computer, this has affected all files normally opened with Daemon Tools.
How can I fix this? I know somewhere there is a... "thing" for setting the file extensions and their association, but I cant find it, and its really being a pain
-Edit
Now, here's something I've found - If I go into Daemon Tools, and tell it to be the file association with .isos, it will cause windows to go back to asking what program I want to open with. Something strange is going on.
Also, I have tried reinstalling Daemon Tools
HKEY_CLASSES_ROOT
. Now reinstall and see what happens. Edit: Ah, I see from your self-answer below that it helped.