In my experience, MSE (Microsoft Security Essentials) is the best antivirus considering price, efficiency and lightness. Unfortunately it is impossible to install it on Windows Server 2012. Microsoft does not allow it for some reason.
Fortunately, Microsoft has another product which is so similar to MSE that you won’t notice the difference. It is part of the System Center 2012 suit from Microsoft. The whole suit is free but weighs 1.5GB so it is quite inconvenient to download it and find the Antivirus inside. So lucky you are, someone extracted the antivirus for you.
Tested, confirmed to work, and it is clean for sure because it is extracted right from a Microsoft archive.
Note: you may want to look at pytex’s comment for an updated version. I did not test it.