AVG is a very good Anti-Virus programs. It also scan for Malware, Ad-ware, and Spyware. Adds a toolbar into your browser to scan for Unsafe websites, Phishing websites, and more. Very few false positives. Web attack blocking.
2. Avast! Free Antivirus 5
This is another good free one. Very good detection rate. Offers a web traffic scan. Decent level of scan speed. However, in most cases it requires you to register for a free code. (It's all free... Just annoying having to jump through hoops to download and install something)
3. Microsoft Security Essentials 2011
Offers real-time protection, decent detection rate, and being from Microsoft, offers the finest level of integration with Windows. Needs minimum amount of computer resources. Good predictive detection as well.
4. Avira AntiVir Personal
Excellent Malware detection and blocking. Quick scan speeds. E-Mail scanner scans sent and received e-mail for malicious files. Needs an interface makeover.
5. Comodo AntiVirus
Effective at blocking new Malware. Includes a firewall. Inconsistent over malicious file detection makes it kind of hard to recommend.