rollovers = {
  'nav_1': ['nav_img_1','images/nav_ro_1.gif','images/nav_1.gif'],
  'nav_2': ['nav_img_2','images/nav_ro_2.gif','images/nav_2.gif'],
  'nav_3': ['nav_img_3','images/nav_ro_3.gif','images/nav_3.gif'],
  'nav_4': ['nav_img_4','images/nav_ro_4.gif','images/nav_4.gif'],
  'nav_5': ['nav_img_5','images/nav_ro_5.gif','images/nav_5.gif'],
  'nav_6': ['nav_img_6','images/nav_ro_6.gif','images/nav_6.gif']
}

if(window)
    window.onload = setupStuff;
else
    document.onload = setupStuff;

function setupStuff()
{
  for(ar in rollovers)
  {
    if(document.getElementById(ar))
    {
      rollovers[ar][3] = new Image();
      rollovers[ar][3].src = rollovers[ar][1];
      document.getElementById(ar).onmouseover = rollover_nav;
      document.getElementById(ar).onmouseout = rollout_nav;
    }
  }
}

function rollover_nav()
{
  document.getElementById(rollovers[this.id][0]).src = rollovers[this.id][1];
}

function rollout_nav()
{
  document.getElementById(rollovers[this.id][0]).src = rollovers[this.id][2];
}