I was thinking much the same thing, Dumper ... firewall blocking the virus database download ... so I turned Comodo & Defense+ off completely (Firefox as well) and went straight for Avast (maintenance) and manual update. Unusual for Comodo to block something without endlessly whining about it and asking what to do next, but never mind.
No joy, though. It went through the same preparatory routine and started to download the database update ... pretty rapidly this time, though, it must be said (13 secs), then claimed the download was complete ... only it can't be, for it tells me I'm still on 130802-2 released on 2/08/2013, while at the same time saying 'Latest update received 17/08/2013' and yes, the smiley face (a bit silly, that, I think), still confirms 'Everything is up to date.'
Either I'm going nuts or this copy of Avast has, somehow, got corrupted and I'll have to go for uninstall/ reinstall ... a bit like scrapping your car because the windscreen wiper is broken ... 'I may be gone for some time' ...