Avast firewall in versions 5 thru 8 was designed to work alongside Windows 7 firewall, while it was recommended to disable WinXP firewall when using Avast firewall.
Avast 2014 firewall has been redesigned. http://forum.avast.com/index.php?topic=136785.msg1000473#msg1000473
•Firewall has been redesigned from scratch to allow for better compatibility with latest Windows, better performance and full support for IPv6 (although its UI hasn’t really changed).