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...and also... it says in it's name... avast! antivirus, so it means it's an antivirus, not a firewall. It is also good thing to install a firewall too to help your computer security.
No, it is not a good thing if you disable Windows XP built-in firewall, especially because you don't have any other firewall product installed. avast! works great with all those "external" firewalls, as well as with Windows XP firewall.