Make Ctrl+Alt+L show login screen

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http://askubuntu.com – I've been messing with some things and the lock screen is not working well at the moment. Since I like the standard login screen much better anyway, I would like to change the action for the Ctrl+Alt+L shortcut. At the moment, it does the same thing as "Lock Screen" in the top-right power/settings menu. I would like it to instead have the same effect as "Switch User Account" on the user menu. How can I do this? FYI, I'm using 12.04 with the classic Gnome session. Things might look somewhat different in Unity. (HowTos)