var OS = navigator.userAgent;
var oLink = document.createElement("link")

if (OS.indexOf("Win") != -1)
{
  if ((OS.indexOf("Windows NT 6.1") != -1))
  {
    document.write("<link rel='stylesheet' href='../css/SevenFont.css' />");
  }
  else if ((OS.indexOf("Windows NT 5.1") != -1) || (OS.indexOf("Windows XP") != -1))
  {
    document.write("<link rel='stylesheet' href='../css/XPFont.css' />");
  }
}
else
{
  document.write("<link rel='stylesheet' href='../css/SevenFont.css' />");
}