ClamAV For Windows is a port of the very popular Open Source antivirus scanner for UNIX/Linux operating systems to the Win32 platform. Using the Cygwin compatibility layerm all major features are implemented (including the Clamd Daemon for very fast scanning, and the FreshClam update tool).  
This comes with an easy to use installer/uninstaller and all the DLL files needed to make it function. There are also several mail server and web proxy applications for that can directly integrate with ClamAV For Windows and provide complete protection for your Windows Server.