that really depends how the definitions are stored. There was a thing a little while back where loads of people thought a publishing company was distributing virusses on a PC magazine CD, it turned out to be a trial copy of Mcaffee, the virus definitions database was scanned andvirus definitions were found.
Norton is probably one of the worst virus scanners ever.
It's only got such a large following because of it's corperate branding, which is mainly grown because of the excellent hard disk tools Pter Norton created before norton became Symantec.
There are hundreds of reports scattered all over the internet from users saying when they changed virus scanner they found loads of virusses that norton failed to pick up.
Do yourself and your computer a favour, dump the resource hungry useless beast that is norton AV.