I don't put much stock in freeware antivirus programs however I have heard several recommendations for AVG and a couple for Avast. I don't have any personal experience with AVG but Avast has kept me safe before I bought a good security package and a few of my less wealthy friends also tend to prefer it. As far as I can tell from the pros AVG SEEMS to be preferable. However on older systems such as 98 it might not work so Avast is another option
As far as paid from personal experience Panda has been best for me, I purchased the titanium firewall and AV combo and have had only one issue I am currently awaiting a response on but it doesn't seem to hog too much memory, I also run firefox and have not run into any conflicts with the programming and to date as far as I can verify with the online virus scans such as Trend Micro Housecall I remain infection free other than the occasional tracking cookies. But as they say, if you don't have tracking cookies, you're not connected to the internet. Also I hear a lot of good user reviews for Kapersky though I have no personal experience with their software. I have had NUMEROUS issues with Norton in the past and adamantly refuse to use anything Symantec.
If you are wondering about the issue I had with Pandasoft it is posted in another thread in this forum and as far as I can tell is nothing serious just an alert I'm having a lot of trouble finding info on