I'd just drop the antivirus and setup some good firewall rules. Unless you're installing new software on the machine regularly or browsing the internet via the Windows server's built in Internet Explorer, it's essentially not necessary. Although I'm not a Windows administrator, so I can't say for sure how essential antivirus is on a Windows server. It just seems useless.
Linux, all you need is a properly configured server, good firewall, and lock out access to areas others don't need.
Anyway to stay on topic, if you do want an antivirus, I swear by Kaspersky when I was using Windows. It's great software as a desktop antivirus, and I'd imagine they have a server version for that as well.