I use ClamAV for Unix/Linux systems. It's a free SourceForge project
that's kept very current. I use clamav on my Postfix server with
amavis to scan all email for viruses and have been _extremely_ impressed
with it's performance. It has a program called "freshclam" to maintain
your virus signature database file with the lastest information. You'll
want to hook up freshclam as a cronjob.
Here's the main page:
http://clamav.elektrapro.com/
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