Actually it can contain a virus still. That is how most viruses infect a system in fact. You download a file from the internet and run it. The file downloads and scans fine but once you run it that is when you get infected cause it runs a separate sub script or something that causes the machine to become infected.
Not to be the burden of bad news, but Norton constantly fails independent tests runs by various sites. It has one of the lowest detection rates out there. That along with McAfee have been struggling to keep up with even free versions of various software out there. Microsoft with their free Security Essentials has been awarded more awards than Norton has as of late. MSE has been awarded Silver when Norton didnt even place in the AV-Comparatives awards last year. So that is something to consider.