I use AVG which is free. I take all precautions I can to avoid viruses like not using Internet Explorer (I personally use Opera the most and then Firefox for the pages Opera cant handle) and not clicking on links in emails and IM's unless I know what I am doing (I use Yahoo Messenger and AIM a lot).
I love AVG and have used it for probably three years at this point. It updates automatically when it needs to and is constantly running. It checks my emails automatically as well. It is really simple to use. I found it through a Tech TV clip when Tech TV was still around (yes I'm a computer geek) and they highly endorsed it at the time.