yeah, its probably the HP proprietary connection for VGA. Is there a white plug near the slots on the computer tower? Most likely you will see it there (It should look something like this)
http://i5.ebayimg.com/03/i/000/ee/e6/aff7_1.JPG . You will need to either get the plug from HP (or whoever gifted it to you) or get a little converter that fits in and converts to whatever you are using. I am assuming its VGA (blue connector).