Would you mind posting a bit on ZoneAlarm 5? I´d be interested to hear what problems it has.
Check around computer forums, you'll get a taste. I think there was a thread or two on this at the Delphi forum. I'd used ZA for years with no problems, and no intrusions, and faithfully upgraded to version 5.xx.
Windows began behaving strangely, little things like icons changing, crashes, weird error messages. Then I ran checkdisk with the fix option, and discovered I was locked out of my C drive - checkdisk gave an error saying the volume was in use, when it shouldn't be. I messed with it for days, and got to discussing it on some forums, and everyone had the same problem - checkdisk could only be run to see if there were errors (run from explorer), but to fix them it reboots and does its thing before loading windows, and it couldn't.
I turned anti-virus and ZA off, but that didn't help.
Uninstalled ZA completely, and suddenly i could fix disk errors and windows stopped freaking out. I posted back to some of the folks having the same problem, and only the uninstall allowed checkdisk to run correctly.
Weird stuff. I swore by ZA, installed it on the machines my friends asked me to build or fix, and no trouble. After seeing this sort of major error, I decided to move on. Tried both Sygate and Kerio - both worked well, but I chose Kerio because I liked the interface. Neither is as easy to use as ZA, bot neither gave me any problems.