UPDATE 20111110: See our updated free antivirus recommendation page.
Here at YCG, we get a lot of requests for recommendations. At the top of the list—as far as quantity of requests—are requests for a free antivirus program.
While every business or corporation should have a professional security package installed and administered by a reputable network administration and security firm (like Your Computer Genius), free antivirus programs may be suitable for the home user.
In the recent past, we have recommended AVG’s free edition, and indeed, we have corporate clients who use the AVG network edition for their security solutions. However, recently it seems like Avast has edged out AVG in terms of ease-of-use, usability, and features.
Here are links to both versions. Please note that you should check the terms of service carefully—many free version of software do not allow commercial use. The summary reviews on CNET are also probably worth a quick glance before you commit to the software.