in etwa so?
getestet und für gut befunden!
<html>
<head>
<script type="text/javascript">
function show()
{
if(document.getElementById)
document.getElementById("vor").style.visibility = "visible";
}
</script>
</head>
<body>
<form name="Formular" method="POST">
<table border=0 align="center">
<tr>
<td ID="nach" class=heading bgcolor=ccccc align="center">
Ihr Nachname
</td>
<td align="center">
<input type="text" onkeydown="show();" name="nachname"/ >
</td>
</tr>
<tr>
<td class=heading bgcolor=ccccc align="center">
Ihr Vorname
</td>
<td ID="vor" style="visibility:hidden" align="center">
<input type="text" name="vorname"/ >
</td>
</tr>
</table>
</form>
</body>
</html>