Avast Software is among the leading free anti-virus and anti-spyware tools out there. Malwarebytes is quick and rarely throws out false positives. BitDefender’s Free Edition acts as an on-demand virus scanner for system recovery and forensics. ESET offers a free Online Scanner offering that detects and removes PC threats. PC Tools AntiVirus Free gives users real-time virus scan and removal to thwart viruses, Trojans, worms, rootkits and other malicious programs.”]
Source: http://www.shoaibyousuf.com/2011/10/10-free-security-tools-that-actually.html