To Lock Desktop using [ctrl]+[Alt]+[Del] key combination

To Lock Desktop using [ctrl]+[Alt]+[Del] key combination:

Just open control panel->User Accounts. Select the action "Change the way users log on or
off". Uncheck the option "Use the Welcome Screen" and click apply. The old security dialog
box will now appear if you click [ctrl]+[Alt]+[Del]. Now press spacebar to lock desktop.

Allow other users to access system services:(Restricting others from access services that
are used by users)