怎样点击后弹出一个固定大小，位于中间，不能最大化的窗体？

Question

<script language="JavaScript">
//to open window in screen center
function openWin(url,w,h)
{
    var px=Math.round((window.screen.availWidth-w)/2);
    var py=Math.round((window.screen.availHeight-100-h)/2);
var mywin=window.open(url,"MyWin","toolbar=no,location=no,menubar=no,resizable=no,top="+py+",left="+px+",width="+w+",height="+h+",scrollbar=no");
}
</script>
<input type="button" value="打开弹出窗口" onclick="openWin('123.htm',300,200)">