Not all the anti-virus software are the same, and it is difficult to know which is the best to use and you may use a few of them. I use AVG and Ad-Aware - a combination which help in a lot of cases. Important is, you may have to do a complete scan regularly to avoid possible infections.
Avoidance may be a good line of defense, by not clicking on any links which are sent to you via email, if they are suspect, even though you may know the sender. Just my opinion. :)