<form name="frmtest">
...
<input type="checkbox" name="dianhuachuli" value=1 onclick="able()">
<textarea name="telneirong" cols="80" rows="8" disabled></textarea>
</form>
<script language="javascript">
function able()
{
 if (document.frmtest.dianhuachuli.checked)
    document.frmtest.telneirong.disabled = false;
 else
    document.frmtest.telneirong.disabled = true;
}
</script>