If kdm is installed and in use. You can enable auto login by opening systemsettings, going to 'login screen', 'convenience' tab, tick 'enable auto-login' change 'pre select user' to 'specified' and select the user you want to auto login. tick 'enable password-less logins' and then tick the name of the desired user.
I don't know what Nagios is, so i can't help you with that. Good luck.