Clam AntiVirus is an open source (GPL) anti-virus toolkit for UNIX, designed especially for e-mail scanning on mail gateways. It provides a number of utilities including a flexible and scalable multi-threaded daemon, a command line scanner and advanced tool for automatic database updates. The core of the package is an anti-virus engine available in a form of shared library. More info »
ClamWin is a distinct app and listed as an alternative to ClamAV.
The link to the Windows version is http://www.clamwin.com
Highly recomended, simplicity, open source, does what it says it does.
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