Kerio is a firewall,it's a developed version of the old Tiny 2. Both are well reviewed in most places. I've them in conjunction with AVG without any problems. But definately as well as, not instead of.
Having read this yet again I think we are talking about 2 different applications, Kerio do an antivirus program that is free to non professional users, which is what I meant. Not a firewall such as Sygate or Zone alarm.