I've used just about every av software out there and all the ones mentioned here are kick ass. I use Avast. I'm not going to suggest what to use but more what not to use. Stay away from McAfee. I installed that a while back because it came free with Comcast cable. It had to be the largest resource hog I've ever had. If the firewall had an issue it wouldn't block traffic like Zone Alarm until it got fixed it would just shut down and you'd be stuck naked on the net. Their AV product was the same too, if it had an issue with an update instead of trying to reload or asking you to reboot it would just turn off virus scan. I'll never go back to McAfee.