Thank you for the reference.
I was hoping, though, for an explanation of why the problem occurs only in mIRC version 6, and not version 5. One would think that if it were a problem with winsock or the system itself, it would happen with either version. If there is some simple misconfiguration I can change that prevents version 6 from working properly, that'd be great.
Just a note - I've tried this on various other computers as well, and asked a few friends about theirs. Every system I've tried that runs mIRC 6 on Windows XP Pro has the same problem, as do all of my friends who run Windows XP Pro (I have not tried Windows XP Home).