<head>
....
<script language="javascript">
function mostrar(tabla)
{
if (eval(tabla).style.visibility == "hidden")
eval(tabla).style.visibility = "visible"
else
eval(tabla).style.visibility = "hidden"
}
</script>
...
</head>
<body>
...
<table id="tablaregistros" onclick="mostrar('tablaregistros')" ...
...
</body>