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Portable ClamWin is the popular ClamWin Antivirus packaged as a portable app, so you can take your antivirus with you to scan files on the go.
ClamWin provides a graphical user interface to the Clam AntiVirus engine.
* High detection rates for viruses and spyware
* Regular virus database updates. [...]

Moon Secure AV is a free Antivirus for Windows under GPL license. It offers multiple scan engines, Net shield, Firewall, On access, on Exec scanner and rootkits preventions plus features from Commercial Antivirus applications.