How to setup an Antivirus for Gateway

We have an existing Email and DNS server all in one Linux box.
Our network is behind a firewall, and we have a mapped IP directed to the Linux Email/DNS machine.
We connect to the internet via a DSL router.
We want to introduce an NT machine thatwill act as an SMTP packet filter and antivirus server.
How do we do this implementation properly without much disruption of service?