You went to FreeNX (http://freenx.berlios.de/download.php) to get the FreeNX server and installed it. It came with a set of default SSH keys. Because everyone in the world gets those, how do you make new ones?
Use /usr/bin/nxkeygen to generate a new key pair. Then, do not forget to copy the new /var/lib/nxserver/home/.ssh/client.id_dsa.key to your client PCs, or they will not be able to log in. On Windows, they go in the \Program Files\NX Client for Windows\Share\ directory. On Linux , Mac OS X, and Solaris they go into /usr/NX/share/.
NX Server System Administrator's Guide: