function showCourse( str ) {
  if ( str == "" ) {
    document.getElementById("txtHint").innerHTML = "";
    return;
  }
  if ( window.XMLHttpRequest ) {
    // code for IE7+, Firefox, Chrome, Opera, Safari
    xmlhttp = new XMLHttpRequest( );
  }
  else {
    // code for IE6, IE5
    xmlhttp = new ActiveXObject( "Microsoft.XMLHTTP" );
  }
  xmlhttp.onreadystatechange = function( ) {
    if ( ( xmlhttp.readyState == 4 ) && 
         ( xmlhttp.status     == 200 ) ) { 
      document.getElementById("txtHint").innerHTML = 
        xmlhttp.responseText;
    }
  }
  xmlhttp.open( "GET", "GetCourse.php?q="+str, true );
  xmlhttp.send( );
}