<script language="javascript">
function underline(eto) {
eval('document.all.'+eto+'.style.textDecoration = "underline"');
eval('document.getElementById("' + eto + '").style.textDecoration = "underline"');
}

function nounderline(eto) {
eval('document.all.'+eto+'.style.textDecoration = "none"');
eval('document.getElementById("' + eto + '").style.textDecoration = "none"');
}
</script>
<a onMouseOver="underline('eto')" onMouseOut="nounderline('eto')">Подчеркнуть</a>
<div id="eto">это</div>