You want to install free antivirus for Microsoft Windows 2012.

Solution

Aside from limited trials, there is no true free antivirus for Microsoft Windows Server 2012 or Windows 2012 R2.

That said, and while Microsoft does not fully support it,  you can install Microsoft Security Essentials on Server 2012, below is how to do so.

1. Download a copy of MSE from Microsoft: https://windows.microsoft.com/en-us/windows/security-essentials-all-versions
2. Right Click on the mseinstall.exe.
3. Click on Properties.
   
4. Click on the Compatibility tab.
5. Locate the Compatibility section.
6. Check Run this program in compatibility mode for
7. Select From the drop down menu Windows 7.
   
8. 4. Open a Command Prompt as Administrator.
9. Navigate to your Downloads folder (ie. cd C:Users%username%Downloads).
10. Run mseinstall /disableoslimit and follow the installer prompts to install MSE on your Windows Server 2012.