I've always used AVG. A little upset with this 8.0 they upgraded to, because the thing I really liked about Avg, is that it doesn't use much to run, but 8.0 is sort of heavy.
I also keep Malwarebytes Anti-malware on the computer, and do a scan maybe once every 2 weeks, and it finds some crazy stuff. If you've ever gotten one of the trojan/virus/horses that change your background to "you're infected!" and a bunch of popups in the notification area near your clock, this will fix you right up.
