This seemed like the answer, since the suggestions in the succeeding response from Syed at MS didn't resolve my issue. Unfortunately, I did the Regedit thing for Acrobat 9, the one probram I'm now trying to set as default,
but "Change Program" in "Set Defaults" still will not accept (or recognize) any programs in the "Program Files (x86)" folder - only those in "Program Files". There's gotta be a straightforward way to fix this - short of reloading the whole operating system.
I suppose the answer might be further down this thread - or in another forum - but this looked like such a logical solution that I hoped it would work.:-(
What did work for me was the following:
STEP 1 - CHECK LOCATION OF THE EXECUTABLE FILE
Check to make sure that Windows knows where the executable file can be found, other wise the file will not open with the correct program when you double click it. You have to use "regedit" fo perform the following fixes
Navigate to:
HKEY_CLASSES_ROOT\Applications and and find your .exe name.
Expand the tree to shell>open>command.
In the Default change its location to the actual location of the executable, hit OK and then try and re-associate the file type as you normally would.
If this does not correct the issue go to Step 2
STEP 2 - CHECK FILE ASSOCIATIONS
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts
Scroll down to your file extent, for example, ".txt" and expand the tree, and select the “OpenWithList” tab. Here you will see one or more programs that Windows thinks your file opens with.
The MRUList gives Windows the order in which the programs will be seeked out to open your file. Look carefully and you may see that there may be more than one selection. In the case of my files, Wordpad was there as well as Notepad but selection “a” was Wordpad
and selection “b” was Notepad.
My MRUList was set to “ab” making Wordpad open the ".txt" file instead of Notepad so I changed the MRUList order to "ba" and now when I double click on any "txt" fle it is opened by Notepad