Oppure, se vuoi essere più sintetico...

codice:
<!doctype html>
<head>
<script type="text/javascript">
function cambiaMarca (oForm) {
	(oForm.modello.disabled = oForm.marca.selectedIndex === 0) || (oForm.modello.selectedIndex = oForm.marca.selectedIndex - 1);
}
</script>
</head>

<body onload="cambiaMarca(document.tuoForm);">
<form name="tuoForm" >
<select  name="marca" size="1" onchange="cambiaMarca(this.form);">
<option selected value="0">tutto</option>
<option  value="1">fiat</option>
<option  value="2">BMW</option>
</select>

<select  name="modello" size="1" ">
<option  value="0">fpanda</option>
<option  value="1">530</option>
</select>
</form>
</body>
</html>