<form name="frm">
<table>
<tr>
<td>
<input type="checkbox" value="1" name="ciccio" onchange="modifica();">
</td>
</tr>
<tr>
<td>
<input type="text" value="ciao" name="franco">
</td>
</tr>
</table>
</form>
<script language="javascript">
function modifica()
{
if (document.frm.ciccio.checked)
{
document.frm.franco.disabled=true;
}
else
{
document.frm.franco.disabled=false;
}

}
</script>