What about buying a 50 bugs Pentium-I used computers(32 Mb RAM, 1Gb HD), and using the last Fedora Core 4 release, or Sarge? You can do that with linux VNCServer. VNC, on linux has the ability of creating desktop environments for each client. Here is an example with FC4.
1. Install vncserver on linux server. Install vncviewer from RealVNC on clients. You need 128Mb RAM on server for each client.
2. Create client entries like the line examples on /etc/sysconfig/vncserver
3. su as each user. Run vncserver. It will create the ~/.vnc structure.
4. Edit ~/.vnc/xstartup:
[ -x /etc/vnc/xstartup ] && exec /etc/vnc/xstartup
[ -r $HOME/.Xresources ] && xrdb $HOME/.Xresources
xsetroot -solid grey
vncconfig -iconic &
xterm -geometry 80x24+10+10 -ls -title "$VNCDESKTOP Desktop" &
exec gnome-session &
5. Check your firewall rules! Start service. Connect from clients to SERVER_IP:5901 (each client is enumerated from 5901).
This method allows you to have a lot of clients working on his own homes. Startup scripts, allowable programs, quotas. But thats a KDE or GNOME issue.