Aqui te va un ejemplito sencillo
Cita: <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Untitled Document</title>
<script language="javascript">
function mostrarForm(valor)
{
document.getElementById('primero').style.visibilit y = "hidden";
if (document.prim.selec[0].selected == true)
{
document.getElementById('soporte').style.visibilit y = "visible";
document.getElementById('info').style.visibility = "hidden";
}
else
{
document.getElementById('info').style.visibility = "visible";
document.getElementById('soporte').style.visibilit y = "hidden";
}
}
</script>
</head>
<body>
<div id="primero">
<form id="prim" name="prim">
<select name="selec" onchange="javascript: mostrarForm('value');">
<option value="0">Soporte</option>
<option value="1">Información</option>
</select>
</form>
</div>
<div id="soporte" style="visibility:hidden;">
<form id="soport">
<label for="sop">Soporte</label><input id="sop"/>
</form>
</div>
<div id="info" style="visibility:hidden;">
<form id="information">
<label for="inf">Información</label><input id="inf"/>
</form>
</div>
</body>
</html>