Ubuntu – not asked for password at startup

passwordsessionstartupxfcexubuntu

I have Xubuntu 11.10.
When I log out and in (without restarting the computer) I am always required to chose a session and provide a password for entering that session.
But when I restart the computer I am logged directly into my last session (or asked to chose between my saved sessions – as an already logged user – , if I check to be prompted on login) without any password being asked.

In all the other normal circumstances I am asked for my password (installing in terminal, accessing Synaptic, using root access etc) – except when opening my computer, which is odd.
In Setting Manager, Sessions and Startup, I see (and have checked) the options regarding the login, logout and saving sessions but there seems to be no option for the startup as such, just for sessions. Is there such an option? There must be, I have the impression that after the first OS installation (some 4 months ago) I was beeing asked for password when starting my PC.
How could I turn that back?

— Considering some answers I got, I add the following:
I cannot find where to set the login window to appear (the one that in Ubuntu is System>Administration>Login Window). If I set to be prompted upon login, as I said (in Setting Manager, Sessions and Startup), that is a different matter: I am in fact already logged in as user when asked to chose between user-saved sessions. I don't seem to find the settings for System–> Login Window so as to check the option to be prompted to chose sessions (even Ubuntu or Lubuntu sessions if installed) AT STARTUP. —

Best Answer

In the file /etc/lightdm/lightdm.conf, comment out the line starting with autologin-user=.

For the record, in 'Applications Menu > System > Users and Groups' you can toggle the option 'Don't ask for password on login'. However, lightdm.conf apparently overrides this on startup. As far as I can tell, there is no way to change this through a GUI.