una cosa del tipo:

codice:
<html>
<head>
<script type="text/javascript">
function enabletext(value) {
    if (value == "paperino") {
        document.getElementById('campoB').disabled = false;
    }
    else {
        document.getElementById('campoB').disabled = true;    
    }
}
</script>
</head>

<body>

<select id="campoA" onchange="enabletext(this.value)">
<option value="pippo" selected="selected">pippo
<option value="pluto">pluto
<option value="paperino">paperino
</select>
<input type="text" id="campoB" disabled="disabled" /> 
</body>
</html>