This is the default behaviour of /window when you give it a side-listbox.
/window -l12 @window
This uses your default mIRC settings, so if you join a new channel and see the listbox on the left side of the window, then mIRC is working as it should.
To change this, right-click on any channel in the switchbar and select 'Nick List...', then select (o) Right and [x] Set As Channel Default.
It would be nice if we had a /nicklist command, though.
- Raccoon