Possible Solution. You could download another antivirus product, a free one, such as ClamWin. This is not to replace Symantec, only to get Symantec to scan the NAS.
Schedule your task to run as a local user.
In the batch file, perform a
net use x:
\\nasserver\share /user:username password
then you can do a
clamscan.exe -r x:\
http://forums.clamwin.com/viewtopic.php?t=800
http://forums.clamwin.com/viewtopic.php?t=1892
If you have Symantec configured to scan network folders, both products will scan your NAS. Symantec should locate a virus though, and not let Clam scan it. As I recall, Clam will extract ZIP files into %temp%, so Symantec could locate viruses after extraction. Really depends on what your real time settings are set to.
This gets you more control over what to scan than what doscan gives you.