function rollon() {
  if (window.event.srcElement.className == "Item") {
     window.event.srcElement.className = "Highlight";
  }
}

Rollover.onmouseover = rollon;

function rolloff() {
  if (window.event.srcElement.className == "Highlight") {
     window.event.srcElement.className = "Item";
  }
}

Rollover.onmouseout = rolloff;