Session - to restrict same user from two logins

Hello: Session: I am using sessions for user authentication in one of our sites. Are there any ways to restrict the same user from logging in from two different machines. That is, once if the user is logged in, and if somebody tries to log-in using the same userid and password it should be able it display a message 'already logged in from other another computer'. I have seen a similar sample in venturewire.com. Please help me to find out a solution for this. Many thks & best rgds Sheni R Meledath sheni@cyber-gear.com