var current, newImages, oldImages;        
newImages = new Array();           
current = -1;

function hoverMenu (id) {          
  if (list = document.getElementById(id)) {    
    images = list.getElementsByTagName('img');
    
    for (var i=0; i<images.length; i++) {  
      img = images[i];
      source = img.src;                   
      current = current<0 ? source.match('_down.') ? i : -1 : current;
      newImages[i] = new Image();                   
      newImages[i].src = current !=i ? source.replace('_up.', '_down.') : source;
      img.num = i;
      
      img.onmouseover = function() { this.src = newImages[this.num].src };
      img.onmouseout = function() { this.src = this.num != current ? newImages[this.num].src.replace('_down.', '_up.') : this.src };
    }
   }    
}             
window.onload=function() {hoverMenu('mainmenu');}