Well, I guess the best way to do it is the shut down the NAV Server service and then simply move the database. When that is done you start the service again.
Start the service, as then start client on the server PC, On open datbase window, eter the the server name and the path of the datnase on the database name field.
If the database is getting large then you need to expand the DB with more database parts. In any case you should never have had your DB on the C: drive.