Algo así es lo que estás necesitando?
<pre>
<html>
<head>
<script language="JavaScript">
function cambiar(){
if (document.f1.sel.selectedIndex == 1);
document.f1.txt.disabled = false;
document.f1.txt.focus();
}
</script>
</head>
<body>
<form name='f1'>
<select name='sel' onchange='cambiar();'>
<option>Deshabilitado</option>
<option>Habilitado</option>
</select>
<br>
<input type='text' name='txt' disabled value='deshabilitado'>
</form>
</body>
</html> </pre>
<hr noshade size=1 color="#000000"><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]