type
  TMyHintWindow = class(THintWindow)
    constructor Create(AOwner: TComponent); override;
  end;
implementation
{....}
constructor TMyHintWindow.Create(AOwner: TComponent);
begin
  inherited Create(AOwner);
  with Canvas.Font do
  begin
    Name := 'Arial';
    Size := Size + 5;
    Style := [fsBold];
  end;
end;

procedure TForm2.FormCreate(Sender: TObject);
begin
  HintWindowClass := TMyHintWindow;
  Application.ShowHint := False;
  Application.ShowHint := True;
end;