I use AVG which is free and is very good. I download updates evreryday and do a complete scan everyday. Can't be too sure.

Every so often (once a month), I go on line to Trendmicro and run PC Cillin House doctor. This is free.
http://housecall.trendmicro.com/housecall/start_corp.asp

I also use Googles toolbar and control popups.