Earlier this year I was using Chatzilla to chat with my friends thru Cairdiuil Network, my group decided to leave that network and come across to IRC.org. so I pointed chatzilla to the IRC server. Later on Everybody persuaded me to d/l Mirc, which to my dismay my original Nick is in use, but wasn't when using Chatzilla.
I want to go back to using Chatzilla as the client ? and I am trying to figure out the network specs to put into Chatzilla's Preferences the group uses avalon.az.us.zirc.org:7000.
when I enter this, chatzilla doesn't give me option to join any groups..any help appreciated

This is a problem with Chatzilla, not mIRC.
I suggest you check with their website and/or help forums.

If you switched NETWORKS and your nick is already in use, guess what? You get to pick a new nick (because someone is already using the one you want).

Neither IRC client is your problem.