As I understand it, Kerio/Sunbelt still has a free mode avaialble, the same as the feature restricted free mode in KPF4 - you might find the old 2.1.5 archived arond, but it's a bit long in the tooth.
I'd choose anything other than Zonalarm, as ZA is such a rotten cleanup job when it goes wrong - having just had to fix a friends system, where it had messed up badly and would neither work properly nor uninstall.
The ONLY thing that worked, was installing a later version, which fixed it - I'd have uninstalled the wretched thing then, if I wasn't afraid of it making another uninstall foulup.
ZA (free) offers very primitive control, as compared to Kerio or some of the lesser known ones.
http://www.firewallguide.com/software.htm - quite a lot here