startList = function() {

if (document.all&&document.getElementById) {
navRoot = document.getElementById("nav");
for (i=0; i<navRoot.childNodes.length; i++) {
node = navRoot.childNodes[i];
if (node.nodeName=="LI") {
node.onmouseover=function() {
this.className+=" over";
  }
  node.onmouseout=function() {
  this.className=this.className.replace(" over", "");
   }
   }
  }
 }
}
window.onload=startList;


<!-- Begin Preloaden Tabs RollOver

image1 = new Image();
image1.src = "tab_Inwoners_Ro.gif";

image2 = new Image();
image2.src = "tab_Politie_Ro.gif";

image3 = new Image();
image3.src = "tab_Bestuurders_Ro.gif";

image4 = new Image();
image4.src = "tab_Pers_Ro.gif";

// End -->

// Foto Rollover functie -->
function movepic(img_name,img_src) {
document[img_name].src=img_src;
}