Detecting one account being accessed from multiple computers at the same time
If you just want to log the user out to prevent multiple logins at the same time, then you could use the Session Limit module.


  Session Limit allows administrators to limit the number of
  simultaneous sessions per user.


Also taken from the project page:


  Assuming the session limit is 1, if a user is logged in to a Drupal site from their work computer and they log in from their home computer, they would be forced to either log off the work computer session, or abort their new login from home.