If you're using Activity Monitor, you might see a process in Activity Monitor that's named "iChatAgent (Not Responding)" in red text, with Mac OS X 10.4 through 10.4.2.
Don't be alarmed. Even though the red text may give the impression that something is amiss, you can safely ignore the message unless you can't log in. Activity Monitor can display this message even when iChatAgent is responding.
Download and install the Mac OS X 10.4.3 Update, or later, to address this issue.