<div id="MyCloseBtn" align="center">
<input type="button" value="Close new window & return to tab view"
    onClick="javascript:
    if (top.location.href.indexOf('main.asp') > 0)
    {
        alert('Page has not been opened in a new window -- nothing to close.');
    } else {
        window.close();}"></div>
</body>
</html>

<script language=javascript>
function hideBtn()
{
    var div_ref = document.all("MyCloseBtn");
    div_ref.style.visibility = "hidden";
}

function showBtn()
{
    var div_ref = document.all("MyCloseBtn");
    div_ref.style.visibility = "visible";
}
</script>