Best things in life are free. And so is free antivirus software. You could save a couple of bucks using freebies like this. Of course if you need a more comprehensive solution, this might not be so good. If you are good in administering your computer and very cautious in downloading stuff from the internet, such free anti-virus software is probably good enough.
However in an office environment where you are administering a bunch of PCs and your co-workers are prone to download any kind of stuffs, then the free anti-virus software may not be good enough.
Anti-virus software should go in conjunction with a good firewall. The build-in Windows firewall is not good enough. And yes there are free firewall softwares too.
Anyway here are some free anti-virus you can consider. Got this list from PC World and is a brief line up. I'm sure there are many more out there.
Avira AntiVir Personal - Superior performance and superior features with ease of use. It offers great protection but using it might be frustrating for non-techie.
Avast Alwil Antivirus home edition - Superior performance and very good features with ease of use. They offer solid protection and some good feature set, unfortunately their interface is a letdown.
AVG 85. Free - Very good performance and superior features with ease of use. It has good interface and good web attack blocking but their defence against malware is below average.
Microsoft Security Essentials Beta - very good performance and very good features and ease of use. New boys in the block and solid contender. Should be out of Beta stage by end of 2009.
PC Tools Antivirus Free Edition - fair performance with very good features and ease of use. However it lets in too many malware for any sort of good protection.
Comodo Internet Security - fair performance with very good features and ease of use. Not good choice. Poor protection and poor detection.
See the table for cross reference of Anti-virus protection.