Free antivirus software

Avira
Avira Operations GmbH & Co. KG. is a German antivirus software company.

Clam AntiVirus
Clam AntiVirus is a free, cross-platform antivirus software tool-kit able to detect many types of malicious software, including viruses.

ClamWin
ClamWin is free, open source antivirus software for Microsoft Windows.

Gateway Anti-Virus
Gateway Anti-Virus allows applications across the enterprise to check files for viruses by providing a SOAP-based virus scanning web service.