Norton Antivirus is good - if it's a fairly new version (2002 or 2003 version of the software) and if it's kept up-to-date. Otherwise, it's useless (as any antivirus software would be in that situation). And you have to pay money every year (or two - you can choose that option) to keep getting the virus updates.
When I started at my current job a couple of months ago, I discovered that all of the PCs were running Norton Antivirus... but the newest version of the software was 4 years old, and none of the virus definition subscriptions had been renewed in 3 years. In other words, there was no antivirus protection. That was my #2 thing to fix, as a matter of fact (#1 thing to fix was the lack of firewall).
A 10-user licensing package for Symantec Antivirus Small Business Edition runs about $430 for one year; they also offer a discount if you purchase two years' subcription up-front. It's worth it.