Welcome to BC!
As a matter of fact, there is a list of free programs here:http://www.bleepingcomputer.com/forums/topic3616.html
Either Avast or AVG, listed with links at the above, are good antiviruses, with definitions updated frequently and as needed. Some people report Avast as a resource hog, and some others have had problems with the new release of AVG7; they are both free, so you could try them out and see which you prefer. Right now I am using AVG7, and used the prior version---with good results---for several years.