I have been using ZA, both the free version and ZA Pro, for a number of years on WIN 98SE, W2K, XP Home and XP Pro, probably on a total of 15-20 machines. I have never had one minute's trouble with it and have never identified a true incompatability. If I used it like a turnkey system, it worked fine. If I tried to apply some more challenging requirements, it worked ok, but made the flexibility of a hardware firewall attractive. I have tried Sygate as well. Had a similar experience in a limited evaluation, but I saw no compelling reason to change. I guess I'm not adding much real information to the discussion, but it seemed to me that ZA was getting banged around a little without anyone saying they had been successful with it.