You can use a single line of Perl for this:
(You will need to get the WWW module if you don't have it)
perl -MCPAN -e 'install Bundle::LWP'
This one line of Perl code will get the latest Symantec anti-virus update. You may want to script this out more to be more flexible with the filename, as the name itself will change on Symantec's site (Ususally, the date part is what you will want to change.)
perl -MLWP::Simple -e "getprint 'http://definitions.symantec.com/defs/20051206-009-i32.exe'" >> update.exe