I've been running Kerio 2.x on XP Pro SP1, and SP2 beta with Avast while having no problems of what you speak of.
I see I'm even referenced a bit here for Kerio 2x, yes its not for novices who are not willing to learn, and are not willing to spend the time to configure it correctly. If I don't feel someone can use it correctly, even with time, I tell them to use ZA/whatever since there have been too many people who want to use it, but are not willing to spend the time to learn. 4x is just a forever-beta bloatware project that should have never left beta a long time ago as it obviously wasn't ready.