what was your previous antivirus program before avast?
have you uninstall that before installing avast?
any third party firewall like comodo,outpost that could block the connection? most likely not this one seens you have internet access as you said in your post but asking anyway.
have you check the update option in the avast interface that is check for updates automatically?
Almost all new computers come with a trial version of a third party AV (Unless it was a special order). Most likely McAfee.
See if this helps: How to Successfully Install Avast: http://goo.gl/VLXde