
ClamWin
http://www.clamwin.com/
now, its not a real-time scanner but, its a good manual/free/open source alternative to the high dollar server level AV solutions.
They offer updates and virus definition updates as well.
I use it on mine and just kick off a scan when ever i disconnect, on next login you see if it caught anything.
I'm sure with a little testing a scheduled task could be made to work. I have not tested though.