Edit in JSFiddle

  $(function () {
      $('#txtFirstName').keydown(function (e) {
          if (e.shiftKey || e.ctrlKey || e.altKey) {
              e.preventDefault();
          } else {
              var key = e.keyCode;
              if (!((key == 8) || (key == 32) || (key == 46) || (key >= 35 && key <= 40) || (key >= 65 && key <= 90))) {
                  e.preventDefault();
              }
          }
      });
  });

<input id="txtFirstName" value="">