The "open with" "choose default program" is not wworking

I have noticed recently that when I right-click and select open with and then choose program, it no longer work. it just pops up saying "this file does not have a program associated with it for performaing this action.please install a program, or if one is already installed, create an association in the default programs control panel". Don't know when it starated happening so don't want to try system restore. Saw someone mentioning about looking in registry under.. hkey_classes_root\unknown but this section "unknown" is completely missing form the registry. Any help would be welcomed, Thanks, Mark.

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