Clickable "Open" file stops working / File associations list?

1. Sometimes, in Windows Explorer, the clickable "Open" function
stops working, notably with files like htm files but sometimes with
even txt files. I double-click and nothing happens. Not big files,
less than 100kb, short names. Just all of sudden, the clickable open
functions stops working and the right-context menu no longer offers
Open or Open With. Actually, those options disappear in the IE File
Menu at the time too. Instead, to view the stupid file, I have to
open the IE browser or Notepad or whatever, and use the application
menu to find and Open the file on my hard drive. It is so irritating.
Right now, for example, all of a sudden I cannot click on any htm
file and open it unless my browser is already open. I can, however,
click on and open txt files. Most of the time, I can double-click any
file and it opens right up. It is annoying to have it just out of the
blue stop working on certain files for a while.

This condition disappears too. Probably after a reboot tho I havent
actually paid enough attention to test that. It is just today that
got so annoyed with this happening that I want to know why it does
this and whether I can repair the problem without having to reboot.
My mouse seems to be working otherwise, I can select and move and all
that. It's that the files won't open.

2. Somewhat related. How can I see a list of all the files types
associated with given applications? It's impossible to do this in
Folder Options. For one thing, all the file types are not even
listed. In fact, many of the most common ones are not. Htm, for
example. If I use the add/new function to "add" the htm extension to
the display list, THEN folder options tells me that htm is being used
by Netscape Markup. l don't even use the Netscape browser and so far
as I know, don't even have it on my system. It's not in the Programs
listed in Control Panel. Why is that extension (and html) reserved
for Netscape? I use Internet Explorer.