If you used to login to Linux using GUI, it means you were using X display manager. There are many display managers. Among popular ones are gdm, kdm, mdm and lightdm. You can change display manager using gui tools but you can also use command for that purpose. On Linux Mint (and i think on other debian-based distros), you can use this command (say you are using gdm):
A dialog will ask you whether you want to change to other installed display managers or not. You can replace mdm with other installed display manager.
That's all there is to it folk. Have fun! :D