Norton Antivirusmay be a resource hog, but the firm which offers it, Symantec, is a reputable and responsible member of the business community.
Now, as Roger points out, an erroneous "Subscription Expired" notification is not an unknown occurrence. SSometimes a simple reboot is all that is required to correct this, sometimes running "Live Update" often resolves the issue, though not in every instance. Sometimes, most often seen when dealing with upgrades to older versions of Norton AV, replacing the Live Update module with the latest version for your configuration (available at Norton's Support and Download pages) will resolve the issue. At the extreme, and assuming you have retained your valid product activation key, fully uninstalling the product and reinstalling it, carefully following the prompts presented during the install process will be necessary. Completely uninstalling NAV and/or other Norton/Symantec products can be a bit tricky; tools to accomplish that for assorted Norton/Symantec products and product bundles are available on Norton/Symantec's sSupport pages.
Now without going into detail on how to do it, as it really is not the right thing to do
I will say there is a relatively simple way - for those familiar with and comfortable playing around in the Windows Registry - to uninstall a currently supported version of just about any Norton/Symantec product (and many other products as well) in such a manner that upon re-installation from the original media and input of the original valid product activation code, one's subscription period begins anew with the date of the reinstallation.