AVG
Link: http://free.avg.com/us-en/free-antivirus-download

Avast
Link: http://www.avast.com/free-antivirus-download

Avira
http://www.avira.com/en/avira-free-antivirus

Bitdefender
Link: http://www.bitdefender.com/solutions/free.html

PcTools
Link: http://www.pctools.com/free-antivirus/

Microsoft Security Essentials
Link: http://www.microsoft.com/en-my/security_essentials/default.aspx