Norton has an odd habit of spontaneously turning its antivirus protection off.
To make matters worse, it often won't turn back on.
To make matters really bad, Norton doesn't have a "repair install" feature - remove completely and then reinstall to get it working again.
But here's the kicker - it almost never uninstalls cleanly!
And if any traces are left behind - it refuses to reinstall, and McAfee (which I think is a far better product) won't do an install either, claiming that Norton's still on there!
If you want a free antivirus that's pretty competent,
www.avast.com is your ticket.
If your Norton is Effed up and won't go back in, Avast has a light enough footprint that it will install, even with Norton's spoor all over your PC.