The definition files can be found there:

http://www.symantec.com/avcenter/download.html

As for today, the file is named: 1206i32.exe.

BTW, I've changed the routine a bit:
$X=ReadProfileString("c:\Program Files\Common Files\Symantec Shared\VirusDefs\Definfo.dat","DefDates","CurDefs")
If @Error=0
$DefDate=SubStr($X,1,8)
Else
; NAV is not installed
EndIf

Regards,
Flavien