// JavaScript Document
function floatMenu(){
  if(!document.getElementById) return;
  var menu = document.getElementById('menu');
  var distY = ((!document.all)?(window.pageYOffset):(document.documentElement)?document.documentElement.scrollTop: document.body.scrollTop);
  if(distY < 10){ // 600 would be the height of your header area, you may need to change this
    distY = document.body.scrollTop; // this is also the height of your header and will keep the menu here instead of at 0
  }
  menu.style.top = distY+"px";
}
window.onscroll = floatMenu;