How to I control a vista lab with an XP machine

I run a homeless shelter where my clients have access to a 4 computer lab that is set up on a wireless network.  My office computer runs XP and the lab computers run vista.  I do have Safekeeper filters on all of the client computers but I need a way that I can monitor the lab computers remotely from my office computer while clients are actively logged into the lab computer.

unfortunately my budget for software and hardware is $0 and I do want my client's to be able to access e-mail and word processing (resumes) and even some just harmless browsing, but I still need to monitor the computers in real time because of the instant messaging they can do without having a messenger clint on the computer.

any suggestions would be helpful

Thank you

I would try using ultra vnc remote control.  Install the server on the Vista desktops and run 4 client sessions on your office machine, you can adjust the window size to have the sessions tiled on your screen. 

http://www.uvnc.com/ 

Dave