There's no option in mIRC to have it start when Windows starts. You'll have to put mIRC in your Windows StartUp directory.
To have it auto connect and join, use the remote that Mentality posted above or you can go into Options (Alt+O), Options and check Connect On Startup. It will connect to the last server you were connected to. Then go into your Favorites (third icon from the left), add your channels and under Options check Join On Connect. I recommend the remote method though. Then you can add multiple networks if you want.
To have it auto minimize go into Options (Alt+O), Display, Options, Tray and check "On Startup Minimize mIRC To Tray".
