There are several very good free AVs available. Many members here have used AVG for some time with success. Which is "best" really means which is best for you, so there is no reason you cannot try in turn each of the free ones to see which you prefer. http://www.bleepingcomputer.com/forums/topic3616.html
At the minimum, you want one that:
1. Provides real-time download protection (including E-mail scanning).
2. Allows full or partial scanning of your hard drive, both scheduled as well as "on-demand."
3. Provides frequent and timely updates to its virus definitions.
4. Is "easy" for you to understand and use.
Edited by jgweed, 26 October 2006 - 11:52 AM.