If you use Zone Alarm and Windows XP, don't install yesterday's MS security update #951748 'cause it breaks your Internet connection. The explanation and workaround is already posted on the ZA forum, see http://tinyurl.com/5k8zjs.
Side note: I posted this on Twitter late last night when I first ran into it and found the answer. After using Twitter for a couple of months, that seems to be where I turn first to post this kind of thing because it's easy to get the word out quickly. I'm still blogging, but some stuff now goes on Twitter first.