if (document.getElementsByTagName){
  document.getElementsByTagName('link')[0].disabled = false;
  //[Optional]
  document.getElementsByTagName('link')[1].disabled = false;
}

function changeCSS(nr)
{
  if (document.getElementsByTagName)
     x = document.getElementsByTagName('link');
  else if (document.all)
     x = document.all.tags('link');
  else
  {
     alert('This script does not work in your browser');
     return;
  }
  nr--;
  for (var i=0;i<x.length;i++)
  {
     dis = !(i == nr);
     x[i].disabled = dis;
  }
}