Microsoft does have an integrated antivirus for Servers. It's very confusing though. It's almost exactly identical to Microsoft Security Essentials. I believe it's 12 dollars/year for 1 Server. It's called Forefront Endpoint Protection 2010. I think they're coming out with a new one called System Center 2012 Endpoint Protection. They keep changing the name. Just make sure you don't get the System Center Configuration Manager version as that's pretty expensive. While it will allow you to manage all the clients. It's something like 1300 dollars.