At home, I use a copy of [http://www.symantec.com/index.jsp|Symantec Antivirus].
I also regularly use the freeware [http://www.safer-networking.org/en/download/|Spybot Search and Destroy]. (If you use Spybot, please consider supporting them by making a donation! They have been working tirelessly on the product for years, always completely for free.) It's saved my computer many times.