Well, I know I once had to install mIRC on 15 pc's, and I would've been happy with an option to skip the dialogs.
But if such a command line parameter were included in the installer, I think you should be able to change the default options with command line parameters as well.
Something like
mirc604.exe /silent /path="c:\program files\mirc\" /autostart
installs mIRC in c:\program files\mirc\ and runs it when installation is complete, whitout prompting except when files are overwritten (/overwrite to skip that too)
