On earlier versions of Windows client and server you could install Microsoft Security Essentials, free antivirus from Microsoft. In Windows 8 client, it is integrated into Windows Defender. However, I see no such option in Windows Server 2012 and attempts to install MSE fail.
Currently the answer is "It's not supported on Server operating systems", as Microsoft doesn't list Server (2012) as a supported OS.
Microsoft Security Essentials is free* and easy to download and use. Here's what you need to run it:
Operating System: Windows XP Service Pack 3 (SP3); Windows Vista (Service Pack 1, or Service Pack 2); Windows 7*
As with previous versions you may be able to unofficially install them at some point, but for now, it's not compatible.
Security Essentials lists only the client operating systems XP, Vista, Win 7 as supported. Windows 8 is very specifically noted as not supported. Since Server 2012 has the same core, it would be normal to assume that it isn't supported.
Windows 8 currently isn't supported. Please see the system requirements page for a complete list of operating systems that Microsoft Security Essentials supports.
Security is important and if you're still having issues making it install or run, you have alternatives such as MS End Point Protection 2012 that downloads as a 1 1/2 GB trial but includes an unlimited and official (scepinstall) Server version of MSE.
When you don't have the bandwidth for a such a heavy download, you might be tempted to download a 3rd party hosted scep that can be as authentic as the official version after you verify the hash with something like fciv or your preferred method: (Microsoft's official checksummer / ref and dl as of today:) http://www.microsoft.com/en-us/download/details.aspx?id=11533 http://www.microsoft.com/en-us/download/confirmation.aspx?id=11533 (long line wrapping)
fciv -both scepinstall.exe
// File Checksum Integrity Verifier version 2.05. //
9aeb45d54695d3d5bbe43620e70c31fe 17c90933a06c8bb15958579d068ad0b7852264e5 scepinstall.exe
Feed any or both hashes into a Microsoft site specific search: https://www.google.com/search?q=9aeb45d54695d3d5bbe43620e70c31fe+17c90933a06c8bb15958579d068ad0b7852264e5&site=microsoft.com
And it results listed and valid.