Yes to uninstalling any current resident anti-virus.
No to disabling your xp firewall (it is better than no firewall), avast isn't a firewall, it does have Network Shield (limited intrusion detection) but not a firewall.
You would be advised to seek out another firewall as windows xp doesn't provide outbound protection. A forum search for avast and firewall should produce some hits as this has been covered many times and there are a couple of polls about firewalls.