xp user log off

I recently installed xp on my computer with 4 different users and only myself as an administrator, all others are restricted. My concern is that when any one of us is logged on and the computer is left alone for about 5 minutes the sceensaver kicks in, we move the mouse to get back in, the pc averts back to the logon screen where any one can logback on. Is this a regedit feature or is it user account feature. Please advice
Thank You Manuel
email address is garzam@bcsd.k12.ca.us