Código:
<form id="form1" name="form1" method="post" action="">
<label>
<select name="select" id="select">
<optiononchange="document.getElementById('text1').disabled=true;">A</option>
<option onclick="document.getElementById('text1').disabled=false;">B</option>
</select>
</label>
<label>
<input type="text" name="text1" id="text1" />
</label>
<form>

