The free ones I'm familiar with are AVG, Avast, and AntiVir. In terms of detection rates, I'd take AntiVir. AntiVir's free version is missing some features, however... for example, the real-time protection on the free version will always ask the user what to do with a suspicious/infected file. And the user can make mistakes. Also, AntiVir will display one splash screen per day, when it updates.