Les and Austin,I don't mean to disagree with your solution, but NAV CE has a limit of 3 concurrent machine updates at one time through the console. If you have a small business that might be okay, but if you have more then a couple hundred this is just not realistic. The batch file method that Norton uses has its own faults and lack of control.
If the clients currently have no Antivirus then you can create a silent package and deliver via SMS or similar tools, or a Logon script. Yes, this would not be able to be done though if the users are not local Admins on the system. Nigel also says he has 95/98 clients to update. Those are quite easy since they will automatically reboot when done and you have no permission problem to deal with.
That is why I worked on the other script pointed out by DrillSergeant. Note though that I still have not tested it in production, and it may still require some tweaking but it can easily be modified if needed.
As far as updating the definition files, I do agree that using the console to update a Parent and then allow the children clients to update is the best method.
Here is a link though for checking the date and trying to "force" an update if the console is having trouble updating the client.
Update NAV Definitions