Kaspersky is better than any of the free anti-virus programs.

from : The 46 Best-ever Freeware Utilities

Best Free Anti-Virus Software Updated February 2, 2008

Antivirus tools are essential in protecting the host client from a multitude of viral, Trojan, and rootkit Internet threats. They also can also stop virus infestation from cross media file sharing. Antivirus is also essential as a secondary preventative Internet security solution beyond a firewall.

If you are looking for the best possible protection my top recommendation is Avira AntiVir Personal Edition Classic [1]. Although its detection rate is outstanding there are some reservations. Firstly, it lacks email scanning that is only available in the paid version. This means that AntiVir won't warn you about any infected emails before you open them. However, should you open an infected email, then AntiVir will still spring into action. The absence of its email scanner doesn't mean you are not protected from email based infections. My second reservation is that AntiVir is quite an intrusive product you will certainly be well aware of its presence. Finally, AntiVir Personal Edition Classic has a time limited license. It is renewable, but be aware you will have to periodically go through the hoops.
Recommended products:

If your not prepared to accept the drawbacks to AntiVir I would suggest either AVG Antivirus 7 Free Edition [2] or the Avast! scanner [3]. Neither is quite as effective in detection as AntiVir, however they are both more complete products and less intrusive in use.

AVG Free has been continuously refined since it was first released in 1991 and the latest version V7.5 makes further improvements to an already solid product. Additionally, it's relatively small, light on resources, has regular automatic updates, and handles email scanning. There is a free and a pro version; the only difference being that the free version has a few non critical features disabled and has no technical support other than a free user forum .

Equally effective is the free Avast scanner [3] although its funky media player style interface is not to everyone's taste. Avast also required periodic re registration while AVG does not. However, Avast does not seem to suffer the signature file update problems that plague some AVG users.

AVG and Avast are excellent free products that will meet the needs of most users, however, none
of these offer the best malware detection available. That title belongs to commercial products like NOD32, F Secure, the full versions of AntiVir, Kaspersky AV and others. They are however capable packages that offer the financially challenged a real alternative to the major anti virus suites.

However, if you use AVG or Avast in conjunction with a sandbox for surfing, (see section 4) and an anti spyware product, (see section 3) you can achieve a level of protection approaching that offered by the best commercial AV products.

Further improvement is possible running regular on demand scans with a different anti virus product.

On demand scans should be run regularly, at least weekly, to check for viruses and other malware that may have been missed by your main scanner.

A good option for on demand scanning is the free version of the commercial AV product BitDefender [4]. It's a first class product with excellent detection rates, but as the free version lacks an email scanner, and a resident virus guard, it's only really suited for use as an on demand scanner rather than your main AV product. There are some other limitations as well. Firstly, it has an annoying habit of detecting malware products that have been quarantined by other security products and you can't exclude these areas from subsequent scans. Secondly, it is only available on a one year non renewable license.
Product Specifications:

[1] AntiVir
Website: Free antivirus - Avira AntiVir

[2] AVG Anti Virus
Website: AVG Free Advisor - Free antivirus and anti-spyware downloads

[3] Avast
Website: Free antivirus - avast! 4 Home Edition

[4] BitDefender
Website: Antivirus software - BitDefender - The future of security now!