I’m pretty sure this has nothing to do with Kerio, because I don’t use it (and I don’t use any other firewall).

But I have the very same problems. Somehow it appears to me that from time to time avast locks files and forgets to unlock them. Symptoms: I cannot delete a file (because XPPro insists the file is being in use) or I cannot delete a Zip folder just after having unzipped it’s contents. However, stopping avast resident scanner in such a situation and the deleting the file always works.