RE:Desmarcar un combo con seleccion multiple ?
<script language="JavaScript">
function deseleccionar(){
for (i=0; i < document.formulario.combo.length; i++){
document.formulario.combo.options[i].selected = false;
}
}
</script>
<form name="formulario">
<select name="combo" size=5 multiple>
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
<option value="4">4</option>
<option value="5">5</option>
</select>
<input type="button" value="Deseleccionar" onclick="javascript:deseleccionar();">
</form>
espero que te sirva de algo :)