hey Brad, when it comes to the free stuff, Avast and AVG are probably the most popular.
they're both...'ok',...probably as good as you will get for free.
Here's some stats from a magazine I trust, Maximum PC, from a year ago when they tested a dozen or so virus protection applications.
Maximum PC - January 09
McAfee Total Protection 2009 - 6
Norton Internet Security 2009 - 9
Kaspersky Internet security 2009 - 8
AVG Internet Security 8,0 - 6
Avast! 4 Home Edition - 4
Avira Antivir - 7
Frisk F-Prot Antivirus - 5
AVG Free Edition - 6
PC Tools Antivirus - 3
ESET Smart Security - 9
Note 1 - some of the above AV programs are free, most require purchase.
Note 2 - take note that many of the companies have multiple offerings, not all of which were tested.
The Norton offering above is a complete code re-write (they don't even use the Windows Installer!) and is significantly an all around better product than previous Norton offerings. I bought it and I have to admit, I really like it, with rarely ever a noticeable PC slow-down, and it IS catching some stuff on things I download.