Are you referring to the free one or the paid version? Is the latest version of ZA the system hog earlier versions were? One of the reasons I dumped ZA was because of the system slowdowns that were rediculous.
If you don't like the way Kerio is being ran after the free trial period, then you should go with Comodo Person Firewall ( http://www.personalfirewall.comodo.com/ ) I personally use it and it's very good, it's like Kerio and ZoneAlarm put together without the annoying messages every 2 seconds and it doesn't use that much resources
Also it's very good if you run a server because you can set a rule for only your server instead of allowing all connections and stuff