I think you should first read the nagios docs again and slowly add one service check on a remote machine by using nsca. The readme that comes with nsca should help you, and also the nagios docs explain what you need to do for passive checks and how to use nsca.
The idea is that there will be a NSCA daemon running on the central nagios server. That daemon listen's for a nsca client. When a nsca client connects it will pass the data it has to the daemon. The daemon will then pass this data to the nagios daemon. Nagios will then parse the data and display it.
Now how the nsca client get's its data is another story, and depends completely on what remote setup you choose to use. I personally use a mini nagios setup on any 'nix type boxes, or Solaris or whatever. But with WinBlows boxes, there are things like NC_Net that you can setup on the remote winblows box that has nsca built right into it.
So download a few of these, read the readme's that come with them, see if it will work for you, and try em out. So far, they are all pretty much complete.