Well, I just double click. I don’t have my vista machine with me, but if I recall it seems as though the Admin account works fine, but my normal user account does not. Obviously notepad was associated with the .php extension at onetime, but for the life of me (and this could be partially due to my lack of experience with vista) I can not make EmEditor to be the default application associated with .php files.
I hope you have resolved this issue by now, but I don’t know why this happens. It might be Windows Vista problem. PHP should not be associated with Notepad when you first install fresh Windows Vista. Later you must have associated PHP with Notepad. I would unregister PHP from Notepad before install EmEditor.