First of all, i know it seems daunting. I came from solely windows myself, and didnt get into linux untill they started it in school. Most book stores have only Red Hat around here too. I have heard that Mandrake is great for beginners. Check Amazon or your favorite online book store. Truthfully, those Red Hat books are how i got started.
thats a link to Gaim, which handles AIM, ICQ and others.
but first, you have to get your network settings configured.
It will take a little bit of work to get your network up and running, and able to browse your windows computers.
the firewall thing...your talking about two different kinds of firewall. Hardware and software. Not really redundant, but rather, more secure. However, i think you'll have to disable it to use Samba, i'm not sure, but when you get to that point you'll find lots of posts around here about that very thing.
first place i'd go...is to amazon and get myself a manual on it. They walk you through quite a bit, and give you an idea of how to use your system. I found a SAMS Teach yourself Mandrake in 24 hours, but your right, for some reason most of the books I've seen for new people are built around using Red Hat. You'll find that there are many similarities between the two though. While some directories may be different, and preinstalled packages may vary, commands and underlying functionality are markedly similar between many distrobutions.
if you run into something technical that you don't understand...well we are here for you!
anyways, I would start by looking around for a book. I installed SuSe first my first time, but switched to Red Hat because of the number of books available on it.
Also, in the upper right hand corner under Main Menu, you'll see Book Reviews. you may find something there.
hope i at least pointed you in the right direction.