horn blows with replacement battery

Car has been sitting outdoors for a couple months without being started or
driven.  Car ran OK when we parked it.  Battery is of unknown age (at least
two years old).  Battery seemed dead when trying to start the van.  I tried
jumping the van with riding mower, horn blew as soon as I connected the
jumper cables.  I put a trickle charger on the old battery.  When the charger
would apply power, the dashboard instruments would cycle.  But after
charging throughout the afternoon, there was not even enough power to
hear the solenoid click.  Went out and purchased a new battery.  
Connected the ground side, and when connecting the positive side, the
horn would blow intermittently and not stop.  Engine would not turn over
(during horn blowing), dash display had the check engine light on, and
electric windows worked.  Disconnected positive side so neighbors would
not shoot me.  Again... the van ran fine when we stopped driving it a couple
months ago.