Please also see the sticky post in this section on making your own security suite for more information

As noted in the article, there are both advantages and disadvantages to using free security sofware instead of a purchased security suite. Personally, I like using some of the freely available tools as they are efficient and as protective as competing products that require purchase.

Still, folks should do their homework and ensure any free products will meet their needs. They should research free product offerings to understand what they will and will not be able to do functionally with these tools.

ADVANTAGES OF FREE SECURITY PRODUCTS
-- Free product offerings are better than having no protection at all (especially for folks on a tight budget)
-- There are actually many great free firewalls, AV products, and anti-spyware tools available (some free products are often as good or better than competing paid products - but you have to do your homework)
-- Sometimes a simple "no frills" solution is all you need and it might even offer better performance than a full featured product offering lots of "whistles and bells"
-- You can try adding a new layer of protection and if you find there's not a compelling need you can uninstall it and it hasn't cost you any money (e.g., if you rarely get spyware and wanted to test out a free product offering)

DISADVANTAGES OF FREE SECURITY PRODUCTS
-- Security suites may cover more areas of exposure for improved protection (so there are no gaps)
-- Some free products may not be as comprehensive in their scope of protection when compared to paid products (e.g., AV protection may be limited to just files and may not cover exploits, rootkits, or other risks)
-- Some free security products may try to upsell folks with occasional popup messages to the more comprehensive paid versions
-- Very limited user support may be available, where full technical support may be available for
-- Most free products are only available for personal use and these must not be used on a free basis in a corporate environment

Below is an analysis of some of the most recent product offerings. Both AVG and Avast have been well rated as basic AV products. They often provide protection for leading edge threats more quickly than even some of the mainstream solutions.

PC Magazine - Updated list of Free Security Software
http://blogs.pcmag.com/securitywatch/2008/05/free_security_software.php
http://www.pcmag.com/article2/0,1759,2304349,00.asp