If you're uncomfortable with changing the firewall settings, a trip to the Apple Store would be a good idea.
If you want to peek, run System Preferences (you'll find it in the dock), click Sharing, then the Firewall tab. You can at least see if the firewall is on or off. If on, that's the problem iChat is having. You can turn it off to see if that fixes iChat. To keep your security high, it's best to leave the firewall on and only open the ports iChat needs.
Apple ought to add "iChat" as a simple checkbox in the list of known applications.