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I doubt that the avast server edition will do what you want at all - so check very carefully before you make any purchase.

To do what you want is more complex than you are thinking - have you not asked the smart tech you know how it is being done? If he tells you it is just ICS and off the shelf Norton on the ICS machine then he is not nearly as smart as you think.

To do what you want to do would require that you to run the central machine as a proxy server into which all the protocol level connections from the other machines would need to run. (In fact this is just what avast does internally on a single machine - avast is the proxy for your POP/SMTP and HTTP accesses). With those protocols running through a single proxy server that is also the network gateway you can get away with running an antivirus product on just the network gateway. This would have nothing to do with using ICS.

There are products out there that would help you do this - here is a link to one:

http://www.entensys.com/products/usergate/antivirus.php