Here are a couple of options:
http://www.realvnc.com/
that's what i use, you can have it to where you can or can't remotely control the keyboard or mouse. I have it off that way the users don't know you're watching them because the mouse never moves, although the little icon is in the system tray.
I've also here good things about ultravnc: http://ultravnc.sourceforge.net
They support file transfer which I believe realvnc doesn't. These are both things where you can remotely view the desktop and run programs on the other comp. Or you could just use the built in one on winxp like cheyenne said, assuming all the comps on your network have winxp on them.
Hope this helped.