I assume that you are looking for a way to REMOTELY install the NAV Client. I do not know of a way to do this, but we utilize a methode similar to Mike's answer. We put the client install application on a Network drive and load it from the client PC's. This does require a visit to each machine.
As far as finding out which PC's are already running the NAV Client, I believe that the NAV Corporate Edition server application has an interface to 'see' all clients on the system. (unfortunately I am not extremely familiar with NAV CE management) Check the Server interface or in a last ditch effort, check the manual!
Hope this helps!