I agree that a scanner of some kind is a must.

Like the others I've used AVG and was happy with it until it started letting Trojans through. I'd never have known I had them if it wasn't for the fact that my emails were being rejected from my friends' computers by their virus scanners, as infected emails.

I tried Norton, but do not like that one. I tried PC-cillin and that was OK.... it's by Trend. At the moment I'm using McAfee and am quite happy with that one.

Another free one I tried is Avast, and I didn't have any problems with that. The only reason I got rid of it is because my DD bought me McAfee as a gift.
http://www.avast.com/eng/download-avast-home.html