<div id="container">
  <div id="basicSearch">...</div>
  <div id="advancedSearch" style="display: none">...</div>
<div>

#container { MARGIN: 0px auto; WIDTH: 970px }
#basicSearch { width:100% }
#advancedSearch{ width:100%;}

var basic = document.getElementById('basicSearch');
var advanced = document.getElementById('advancedSearch');
if (showAdvanced) {
	advanced.style.display = '';
	basic.style.display = 'none';
} else {
	basic.style.display = '';
	advanced.style.display = 'none';
}