Puoi fare cosi

codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

<html>
<head>
<title>Untitled</title>
<script language="JavaScript" type="text/javascript">
<!--
function setInitialValues() {
 oColl = document.getElementsByTagName('SELECT');
 len   = oColl.length;
 for (i=0;i<len;i++) {
  curSelect = oColl.item(i);
	curSelect.setAttribute('initialIndex',curSelect.selectedIndex);
	curSelect.setAttribute('initialValue',curSelect.value);
 } // for (i=0;i<len;i++) 
} // function setInitialValues()

function ripristina(aSel) {
 aSel.selectedIndex = aSel.getAttribute('initialIndex');
} // function ripristina(aSel)
//-->
</script>

</head>
<body onload="setInitialValues()">
<select id="sel1">
 <option value="0">Zero</option>
 <option value="1">Uno</option>
 <option value="2">Due</option>
 <option value="3" selected="true">Tre</option>
 <option value="4">Quattro</option>
 <option value="5">Cinque</option>
</select> 

<input type="button" value="ripristina" onclick="ripristina(document.getElementById('sel1'))" />
<hr>
<select id="sel2">
 <option value="0">Zero</option>
 <option value="1">Uno</option>
 <option value="2">Due</option>
 <option value="3">Tre</option>
 <option value="4">Quattro</option>
 <option value="5" selected="true">Cinque</option>
</select> 

<input type="button" value="ripristina" onclick="ripristina(document.getElementById('sel2'))" />
<hr>
<select id="sel3">
 <option value="0">Zero</option>
 <option value="1">Uno</option>
 <option value="2">Due</option>
 <option value="3">Tre</option>
 <option value="4">Quattro</option>
 <option value="5">Cinque</option>
</select> 

<input type="button" value="ripristina" onclick="ripristina(document.getElementById('sel3'))" />
</body>
</html>