For those running W2003 / W2008 server Virtual machines on a Windows 7 Host Microsoft Security Essentials doesn't work - you get a Not designed for this operating system message.
Pity - but I suppose MS thinks if you are running a server you are likely to be a business which can pay MONEY for AV software.
However Malwarebytes antimalware does work.
I'm not at ALL worried about viruses -- never had any on HOME (not WORK computers - different matter) any more so most conventiional AV software can go the way of the DODO as far as I am comcerened.
Malware however is a FAR more insidious threat - much nastier and more difficult to deal with - especially drive by infection types as you never know when clicking on a web site link where the next link will take you to.
I know a lot of non business users test Server software out so I hope MS makes a version available for its server systems.