A. You can get the security dialog box back by disabling the Welcome screen. Select Control Panel, User accounts, Change the way users log on or off, and then clear the "Use the Welcome screen" check box.

If you want only the Lock Computer option on the Task Manager Shut Down menu, you can disable Fast User Switching. Select Control Panel, User accounts, Change the way users log on or off, and then clear the "Fast User Switching" check box.

If the Welcome screen and Fast User Switching are both enabled, Switch User (Winkey+L) returns you to the Welcome screen (essentially locking the computer). Someone else can log on to his or her session, but your session remains safely locked.