Below is a partial list of some antivirus products we have worked with over the years and recommend.
AVG
AVG is free for home and personal and sells a great commercial product as well. It is both an antispyware and antivirus product. AVG's footprint is much smaller than some of the big-name AV companies. Significant performance increases can often be seen after switching. It consumes roughly half the system memory that Norton products do giving you back valuable system resources.
When the definitions are out of date the system tray/taskbar icon will show in a grey scale giving you a visual cue that something needs to be done. The free version does not have a firewall but I still whole heartedly recommend this European product.
Be careful of fake download sites! AVG's web site is www.free.avg.com Recently sponsored links on google have mislead people to download fake antivirus programs.
AVAST
Another great antivirus product is AVAST. AVAST provides a 60 day free trial at this point. AVAST can be downloaded at http://www.avast.com/.