If you've eg. Xubuntu on your server with Xfce as the default desktop environment, then it's not trivial how you can connect with FreeNX and use this desktop environment. In the nxclient enter the session configuration screen, set the "desktop" to "Unix" and "Custom", enter "Settings" and choose the "Run the following command" for the "Application", and enter this into the textbox: /usr/bin/xfce4-session. In the "Options" select the "New virtual desktop". Press OK and select in the "Display" section "Full screen"