An administrator can enable and configure virus scanners for use on uploaded files via Site administration > Plugins > Antivirus plugins > Manage antivirus plugins.
ClamAV antivirus is available as a standard plugin. It's likely that more antivirus plugins will be available in future from the Moodle plugins directory.
To make use of ClamAV antivirus, ClamAV® should be installed on your server. See http://www.clamav.net for more information.