With the installation of ubuntu 11, you’ll find guest login. Which can be used by anyone, although your files are safe, if you wish to block guest login you need to add guest false in /etc/lightdm/lightdm.conf.
sudo vi /etc/lightdm/lightdm.conf
#add following line at the end of file.
allow-guest=false
Now after reboot, you’ll not find option for Guest login.
