Monitor client workstations

We have several instructional labs with 30 winxp pro workstations. Each lab
has an instructor workstation. I'm looking for an application that would
allow the instructor to shadow a student workstation without the student
knowing. Also, it would be helpful if the instructor could take control over
a workstation in the event he need to assist a student with a problem or
prevent the student from playing around during class. Thanks for the
information.