By default, when you boot up the computer, Ubuntu will give you a login screen before you get to your X session. Many users are used to just being logged in automatically, and don't want to fuss with logging in each time they reboot their desktop.
To set this in Ubuntu, go to System -> Administration -> Login Window. You'll need to provide your password, then you'll get the Login Window Preferences window with five tabs. Choose the Security tab and click Enable Automatic Login. If you have more than one regular user, make sure to specify which user should be logged in automatically

http://www.linux.com/article.pl?sid=06/06/08/1651225