Sorry for a newbie question......I chat in about 5 channels. How can I set up different nick for different channel?

Thanks!

You can't unless they're all on different servers.

If they're on different servers, you need to use the -i switch in server whilst connecting:

I'll just add that you can run mIRC multiple times at once with different nicks as long as the network lets you connect 5 times at once (many won't).