Free Windows antivirus software, licensed under GPL with open source code. Project information, screenshots, download, news, and user guide... 

http://www.clamwin.com/

Antivirus and security software products for home and business users. Free trial downloads. Virus and malware protection with firewall, anti-spyware, identity protection modules... 

http://www.avg.com

Antivirus software for home or business. The world's fastest antivirus updates. Free virus scan and antivirus trial downloads... 

http://www.kaspersky.com/

Produces the McAfee VirusScan line of antivirus software, as well as various specialized products for home and business users. Also offers managed security services for home and business users... 

http://www.mcafee.com/

Norton AntiVirus, Internet security, and anti-spyware products for the home. Solutions to manage IT risk and maximize IT performance for business. Download free product trials of our fast, high-performing software... 

http://www.symantec.com/

Toolkit for UNIX systems released under GPL. Provides a scanning daemon intended primarily for mailserver integration, command line scanner for on-demand scanning, and update tool... 

http://www.clamav.net/