[Lynne]

AdminCP --> Settings --> Options --> User Registration Options --> Automatic Thread Subscription Mode --> Instant Notification. That will send a message to the thread starter when someone replies to their thread. This setting is for NEW registered users, not users already registered.

To change it for existing users, admincp > maintenance > execute sql query > select query from dropdown. That will set the default for any new threads a user starts. They can still set it to something else themselves though.