Algo así es lo que estás buscando?
Código:
<html>
<head>
<script language="JavaScript">
function cambiar(){
if (document.f1.sel1.selectedIndex == 1){
document.f1.sel.disabled = false;
document.f1.sel.focus();
}
else {
document.f1.sel.disabled = true;
}
}
</script>
</head>
<body>
<form name='f1'>
<select name='sel1' onChange="cambiar();">
<option value="Valor uno">Valor uno</option>
<option value="Valor dos">Valor dos</option>
</select><br>
<select name='sel' disabled>
<option value="Valor uno">Valor uno</option>
<option value="Valor dos">Valor dos</option>
</select><br>
</form>
</body>
</html>
<hr noshade size=1><img src="http://www.gograph.com/Images-8712/ClipArt/cat03.gif" height="50" border=0 align="absmiddle"> <font size="2" face="verdana" color="#000000">
bet[/CODE]