This has nothing to do with whether mIRC is a registered copy or not. You were receiving messages from the channel services on the server you were joining. I haven't seen any servers that don't let you join if your nick isn't registered, but I haven't been on many different servers. If you got this message when trying to join a channel, then you'd need to register your nick to get into that channel. Try joining another channel and see if you have the same problem.