<select OnChange="changeVisibility(this.value)" id="selector">
<option selected value="obj1">Hello</option>
<option value="obj2">Goodbye</option>
</select>

<div id='obj1' style='display:none;'>
Hello
</div>

<div id='obj2' style='display:none;'>
Goodbye
</div>



<script language="javascript">
function changeObjVis(object, value) {
	if (object && object.style && object.style.display) {
		object.style.display = object.id == value ? "block" : "none";
	}
}
function changeVisibility(value) {
	changeObjVis(window.document.getElementById('obj1'), value);
	changeObjVis(window.document.getElementById('obj2'), value);
}

changeVisibility(window.document.getElementById('selector').value);
</script>