huge VirusDefs Directory on my laptop

I am in the process of moving from an old laptop to a new one.  The old one runs WinXPand also runs NAV 2006, which I've renewed and kept updated for many years.  In the Symantec Shared directory, there is a subdirectory called VirusDefs which is over 27Gb in size.  There are several hundred subdirectories in there, most of which contain files starting with the name "tmp...", and they range in size from 0MB up to 327Mb.  They seem to be the results of all of the recent Virus Scans.  Does anyone know if I can safely delete all of these files manually, or can Norton do that for me?  Why does Norton allow this file directory to grow to be so large?