Avira AntiVir Personal Edition, from http://www.free-av.com, is excellent and free as well. I’ve been using it for 5 years on 7 different computers with great success – I’ve also used it to clean several ‘chareidi’ computers (lke, Windows XP with no security software, no updates because no internet, and tens of USB flash drives and MP3 players being plugged in all the time) – it found and cleaned 260 viruses on one PC.
Windows Firewall + Avira AntiVir Personal Edition = no problems, ever.