Check with internet company to find out if it has expired. If it was good for 3 months and you've had the internet service for 4, it's expired. Either put out some money or get a good freebie.

http://www.avast.com/eng/free_software.html

Uninstall NAV if you decide to install Avast. If you want to stay with NAV and you're still within the grace period perhaps a re-install of NAV is required; talk to ISP before you do this. Do you have a NAV cd and serial number ? Check with ISP.

Launch NAV - as in bring up the interface but don't scan for viruses. Click on Status. It should display the renewal date. You can click on it and renew the subscription over the internet.

If you do keep NAV, go to Add/Remove Programs (in the Control Panel) and check to see if your LiveUpdate is version 2.5 or later.  If not you should get the new version (updated for SP2) from Symantec: http://www.symantec.com/techsupp/files/lu/lu.html , download and install it.

The security center may also be mis-reading the product.  Symantec has some fixes for the, but for the most part they will ALWAYS tell the security center that everything is "green" even if there is a problem

Bottomline, check your symantec software.  If it looks good, you're good.  You can install a patch if you can find one, but it isn't going to make the security center any more reliable -- at least not yet