Once in a while (not often), the ClamWin update will fail to work. The various Clam AV mirror sites all have to be syncronized before you can update, and if there is a recent update, it may take a while for the mirrors to synch. There can be other reasons why an update fails, but this is the most common one. All Clam AV virus signatures are prepared manually, and there is no regular update schedule. There does not seem to be any sigmaker who works full-time on Clam now. Some of the Sourcefire people work Clam signatures when their other duties permit, and there are a couple of open source sigmakers who work when they can. There will not be many updates on weekends (Friday thru Sunday), maybe none.
Here is some info on updates at
http://www.clamav.net/lang/en/mirror-problem/ on the web.
More info (a little old maybe) on updates at
http://www.gossamer-threads.com/lists/clamav/users/24251 on the web.
More info is at
http://www.gossamer-threads.com/lists/clamav/users/50228 on the web.
Firewall tips at
http://beginlinux.com/server/firewalls/creating-firewall-rules-with-freshclam on the web.
Regards,