Those people really don't know much. What they are saying is true for AV programs and maybe for other third party Firewalls but not for the Avast Firewall. There are absolutely no conflicts between it and the Windows firewall and Avast actually recommends that you leave the Windows firewall on. I have AIS installed in both Win 7 and XP and the Windows firewalls on both machines are also turned on with no problems whatsoever.
If you set the Avast firewall to auto-decide, it is very much advisable to have the Windows firewall on to better control incoming connection attempts. On auto-decide, the Avast firewall will allow just about everything and really isn't very strong in my opinion. Auto-decide is good for when you first install AIS and want rules for all your installed applications to be easily created when you open them the first time. It can be used like the learning mode of other firewalls. I would recommend, though, that after that initial period, you set it to Ask so that any connection attempts by new things or unknown things will ask for a decision if they should be allowed to connect or not. On Ask, you will get prompts for all connection attempts, both incoming and outgoing as well as prompts from the Windows firewall (if it is on) for incoming. The Windows firewall will also take precedence over Avast's. For instance, if you allow an incoming connection in the Avast firewall but not in the Windows firewall, the connection will be blocked.
