Trend Micro is a good free on-line scanner but I don't think they offer a d/l version that scans in real time. Try it out here:
http://housecall.trendmicro.com/. You have to use Internet Explorer Browser to use it.
Avast and
AVG Anti-Virus are two free AVs that are supposed to be good and they offer real time protection.
Search for more at
Download.com