Due to the potential for conflicts and problems that can arise from using multiple firewalls, ZoneAlarm recommends the use of only ZoneAlarm and a virus scanner.
When multiple software firewalls are installed, there is no way of ordering which product will get a packet and which will report it; the determination as to which product gets operating system preference to monitor the traffic will be made at random. When running more than one firewall, ZoneAlarm might not be able to report on incoming port scans or hack attempts. In some cases, running two software firewalls at the same time can cause more problems than running only one.
ZoneAlarm works well with hardware firewalls - in these cases the hardware firewall would protect incoming ONLY (you would need to set that up for logging if you wish to see those alerts) while ZoneAlarm continues to protect with Outbound Control. Also, note that some software firewalls, such as the built-in Windows XP firewall) provide inbound protection only.
ZoneAlarm provides both inbound protection and outbound protection via application control.