There are many Antivirus products available, it is hard to decifer what they do and what makes them better. Ultimatly they all do the same things and some have other features then others. 

Top 10 Commercial Antivirus Programs

http://anti-virus-software-review.toptenreviews.com/ This site compares the features and gives a rating.

Free Antivirus Programs

Free does not nessisarily mean worse, the free programs are comparible to the commercial ones and if cost is an issue they are a good way to go.

Avast Free Eddition - Make sure you register after you install.  

AVG Free Eddition