Unless they redesigned the file association mechanism completely, it should still be much the same method - look under HKEY_CLASSES_ROOT\.avi and find the default value in the right pane. Scroll down until you find that default value (most likely 'avifile'). Open it, and under the shellex subkey, then PropertySheetHandlers, ContextMenuHandlers etc. subkeys should be the GUIDs of the appropriate handlers. Copy those down just in case, then delete them to disable.