2 antivirus programs that have been found to be quite effective
are :
AVG
www.grisoft.com
free,automatic updates, and seems to work quite well.
AVP
www.free-av.com
another free one with automatic updates. Also has a worm detector. This 1 says not to run more than 1 antivirus program, but it workswell with AVG.
Have seen instances where if 1 didn't catch something the other 1 would, and have seen both pop up on something suspious.