Here is a guide that will show you how to create a login account for XBMC. It will allow you to select XBMC when you login and after entering your password XBMC will started automatically.
To create the account and to make it show in your login screen you need to add two files. To make the files open a terminal window (press Ctrl+Alt+T) and copy+paste the following line:
sudo gedit /usr/share/xsessions/xbmc.desktop
When the text file is opened copy+paste the following lines:
When the text is added save and close the xbmc.desktop file and now you need to create the file for the XBMC session. In your terminal window copy+paste the following line:
sudo gedit /usr/bin/xbmc-session
Now copy+paste the following lines:
# add some initialization stuff here (if necessary)
Finally make the session script executable, copy+paste the following line in your terminal window:
sudo chmod +x /usr/bin/xbmc-session
Now logout and select XBMC from your login manager’s session list.