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Best anti-virus solution for combating LAN viruses? - anti spam linux server

In our institute there are about 60 computers that are connected via LAN. The Internet connection is available from VSAT (satellite link). We are a mail server (Linux), Squid proxy server (Unix) servers, Sun (firewall) and a web server (Windows). Most of the time the Internet connection is very bad. There was no problem at the end of the ISP. We believe that viruses from the Internet represents the largest proportion of the slow navigation. In this context, I know who are the best anti-virus for our system in the following cases:

1. Installed at the end of the server (Linux) - the anti-virus solution, the (mail server or proxy) sun?

2. The End User (Windows) - Software Anti-Virus is the most effective?

Did we need no software anti-spam mail (and other) server install (s)?