Hi Everyone
Avast has been recommended to me and I will install it tonight. I understand that I will have to remove any existing anti virus software that is on my machine. So i also have to disable the xp firewall, or should I leave this active.
Many Thanks
Brian
Welcome mcleodbj to the forum.
Leave your firewall active, it will not interfere with install. In fact you might consider a third party firewall that controls outbound transmissions. 
Yes,be sure to uninstall other av first.
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.
Brian,
Other’s might argue with my choice, but I have had good luck with the free firewall from Zone Alarm.