Gee. I was "fat, dumb, and happy" until I read this topic, but now I am becoming concerned. You see, the latest Avast upgrade (4.6.652) is not presenting any incompatibility issues with my ZoneAlarm, BUT... my ZoneAlarm is version 2.6.088 from May of 2001.
Information from a firewall forum pointed out that there were many problems with ZoneAlarm's August-2001 upgrade, so I decided at that time to leave well enough alone and remain content with v2.6.088 (I am still running Win98SE, so obviously do not need the XP enhancements).
Well, I just checked their history, and ZoneAlarm has had over TWENTY upgrades since my earlier concern. Me thinks I should change to another vendor's firewall.