function doDialog(width,height){
   url = "rulez.php";
if (navigator.appVersion.substring (0,1) > 3){
   var x = (screen.availWidth - width)/2;
   var y = (screen.availHeight - height)/2;
   }
   else{
       var x1 = (800 - width)/2;
       var y1 = (600 - height)/2;
       }

   if (window.showModalDialog) {
   win = window.showModalDialog(url, window, "dialogHeight:"+height+"px; dialogWidth:"+width+"px; dialogLeft:"+x1+"px; dialogTop:"+y1+"px; center:1; help:0; resizable:1; status:0");
   }
   else{
       loadwindow(url,width,height,x1,y1);
   }
}