[CentOS] can I use CentOS as a antivirus / spam filter / HTTP AV gateway?

Hi all

I've been thinking about using CentOS on a Dell R200 server and turn it into a firewall / network monitor / traffic shaper in our datacentre, instead of using a dedicated firewall device.

One of the devices that I have been looking at, with my limited budget, is the D-Link DFL 860 - http://www.netdefend.eu/Product.aspx?m=15&ref=DFL-860

It provides AV, SPI, VPN, DOS, P2P, etc protection. Most of this can be done with Linux as well, but I'm not 100% sure about the AV part.

How will I use / setup CentOS to check all traffic coming in & out (HTTP, SMTP, POP3, IMAP, etc) for virusses and clean them? We host both Windows & Linux servers, and I'm not too worried about the Linux servers, but Windows needs a lot of extra protection.