Selection between Automated and manual testing is decided by the tester according to the requirement of the software interaction with the user or with its internal database.
e.g. If we want to check the login page of the software it involves user interaction with the software therefore now testing can be done manually.
If the situation comes to check the behavior of the software with different values of the database with dynamically changing requirement , you should go for automated testing tools like win runner , QTP etc.