You can write a batch file and then use the Windows Task Scheduler to run it
at a specified time.
If you're running the Norton Corporate edition it can look something like
this:
@echo off
cls
color 0a
Connecting to the dial-up server...
rasdial connectionnamegoeshere usernamegoeshere passwordgoeshere
if ERRORLEVEL 0 GOTO UpdateDefs
color 0c
echo There was a problem dialing out....
:UpdateDefs
C:
cd program files
cd navnt
Updating Virus Definitions...
vpdn_lu /SILENT
Exit