I've been running this FREE version since 2/7/2005. I know that because my free version don't expire until 2/7/2006. I'm serious! The FREE version allows for auto updates, which from what I've seen happen often. The FREE version also allows for scheduled unattended scans. The only thing it does not have that you can get with a paid version is the realtime protection.
Download or learn more here:
http://www.bitdefender.com/bd/site/downloads.php?menu_id=21