ESET NOD32 Free Antivirus

As far as reputations go, ESET's NOD32 is practically unimpeachable. Version 4 of the popular and effective antivirus and antispyware builds on that rep, emphasizing its community-sourced ThreatSense for preventing attacks before they occur, a slick interface, and some of the highest independently-tested detection rates married to low rates of false positives.

AVG Anti-Virus Free Edition

AVG Free provides the bare necessities when it comes to security, but that should be enough for savvy Windows users. You'll get a combined antivirus and antimalware engine, LinkScanner, and e-mail scanning. AVG Free 9 introduces a few new features, with improvements focused on performance, including claims of faster scan and boot times. One new feature is the Identity Theft Recovery Unit. Only for users in the United States, ITRU is a business partnership with Identity Guard which provides "consumer identity theft solutions," accessible only from the AVG toolbar in Firefox and Internet Explorer.

Avast Free Antivirus

Avast Free 5, an A-list freeware antivirus app, provides the same steadfast protection of well-known, pricier antivirus programs. Avast is remarkable for both its effectiveness and arguably providing the most complete free antivirus on the market.
Avast Free 5, formerly known as the Home Edition, protects you with multiple guards. The antivirus, antispyware, and heuristics engines form a security core that also includes multiple real-time shields. The adjustable mail and file system shields join the pre-existing behavior, network, instant messaging, peer-to-peer, and Web shields. The behavioral shield is a common-sense feature, as security software publishers leverage their large user bases to detect threats early and warn others. Other new features include a silent-gaming mode

Avira AntiVir Personal - Free Antivirus 9.0.0.418

Freeware antivirus that provides effective protection against computer viruses

AntiVir Personal Edition is a software that helps you protect and monitor your PC.

AntiVir software is a reliable free antivirus solution, that constantly and rapidly scans your computer for malicious programs (such as viruses, Trojans, backdoor programs, hoaxes, worms, dialers etc.), monitoring every action executed by the user or by the operating system and being able to react promptly when a malicious program is detected.