What you are looking for is the display manager--AKA login manager. The typical ones are GDM, KDM, and XDM. If Linux Mint is like Ubuntu, it will have GDM. When you boot up, the window which appears (asking for your login name and password) should have an option for selecting what kind of session you want.
To verify if a display manager is running (and which one), open a terminal and enter "ps -e|grep dm".