I used to use ICQ regularly until recently. I think my first number was 447*** , which is pretty old. Icq is so bloated now I refuse to use it. I was about to install it again the other day until I came across some hacking/cracking sites for ICQ(looking for ways to block the ads) which scared me away from it. I know theres ways to do things to people on other IM programs but with the way ICQ is now, its even easier for people to exploit it.
kr0m: Don't give up on ICQ just yet!
personally I believe that ICQ is the best IM network, with the worst client. So why user the client? Here's a great alternative I've been using for over a year: http://miranda-icq.sourceforge.net/
In response to the original post, you have to:
a) configure ICQ to use specific ports (instead of random)
b) allow these ports in the firewall
I haven't tried this myself, these are just quickly faked to give you the idea so hopefully that will help: