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#11
Which file type(s) have you been changing?
What happens if you (e.g.) swap txt between wordpad and notepad?
Noted that from the CP 'associate a file type or protocol' => the same (or similar) dialogue to that used in 'Open with' to choose the program. Is that how you set the association manually?
It may be that if the file types you are dealing with are somehow associated with a program that reasserts what it thinks are its defaults if they are not when run, that might give rise to some odd symptoms. A wild guess.
It would seem unlikely that the changes made to the registry are not being saved and lost on restart, as why should that apply to file type associations and nothing else?
I would suggest trying this in
a. Safe Mode - change and restart to safe mode, check again.
If that works:
b: repeat with a clean boot
If that works,
c. continue with a process of elimination.