<!--[if lt IE 7]>
 function alphaBackgrounds(){
   var rslt = navigator.appVersion.match(/MSIE (\d+\.\d+)/, '');
   var itsAllGood = (rslt != null && Number(rslt[1]) >= 5.5 && Number(rslt[1]) < 7);
   for (i=0; i<document.all.length; i++){
     var bg = document.all[i].currentStyle.backgroundImage;     
     if (itsAllGood && bg){
       if (bg.match(/\.png/i) != null || bg.match(/\main_nav_bg.jpg/i) != null){
      	 if (bg.match(/\main_nav_bg.jpg/i) != null) {
      	 	document.all[i].style.height = '47px';
      	 }
         var mypng = bg.substring(5, bg.length-6);
         document.all[i].style.filter = "progid:DXImageTransform.Microsoft.AlphaImageLoader(src='"+mypng+".png', sizingMethod='scale')";
         document.all[i].style.backgroundImage = "URL('../images/x.gif')";
       }
     }
   }
}

window.attachEvent("onload", alphaBackgrounds);
<!--<![endif]-->