Mozilla Firefox on Windows XP won't update antivirus!

I just installed the Comodo firewall and anti-virus, but it won’t update: I have a good DSL connection, always on, but when I try to update the AV I get a message telling that it hasn’t worked, and that i should check my connection. The email and the web work just fine, and I’ll have the internet open on another window, but it still won’t download.

                           ??? -- Hezagzr