function checkOS(){
   if (navigator.userAgent.indexOf('Win') != -1)
      var OpSys = "NT";
   else if (navigator.userAgent.indexOf('Mac') != -1)
      var OpSys = "Macintosh";
   else if (navigator.userAgent.indexOf('UNIX') != -1)
      var OpSys = "UNIX";
   else if (navigator.userAgent.indexOf('Linux') != -1)
      var OpSys = "UNIX";
   else var OpSys = "?";
   return OpSys;
}