Remove this line:
div.footer p.copyright { float: left; margin-left:156px;}

Change this:
div.footer { margin:0px auto; width:720px; color: #000; clear: both; padding: 5px;}

to this:

div.footer { margin:0px auto; width:720px; color: #000; clear: both; padding: 5px; margin-bottom: 10px;}