ClamWin
[Please note that ClamWin Free Antivirus does not include an on-access real-time scanner. You need to manually scan a file in order to detect a virus or spyware.] ClamWin Free Antivirus comes with an easy installer and open source code. High detection rates for viruses and spyware; 32.4 MB download.
It features: High detection rates for viruses and spyware; Scanning Scheduler; Automatic downloads of regularly updated Virus Database. Standalone virus scanner and right-click menu integration to Microsoft Windows Explorer; Addin to Microsoft Outlook to remove virus-infected attachments automatically. Available in 7 languages. Available also in a portable version: http://portableapps.com/apps/utilities/clamwin_portable
