From either Explorer or My Computer click on View | Options, then the 'File Types' tab. Scroll down the list of registered file type until you find 'Registration Entries', highlight it and click 'Edit'. Listed in the 'Actions' box should be three actions: Edit, Merge and Print. Highlight 'Edit' and click 'Set Default', Edit should become Bold.
Next time you double-click on a .REG file, notepad should open instead. You can still import a file by simply right-click on it and select 'Merge'.
