Автор: http://www.swissdelphicenter.ch

type
  TXPHintWindow = class(THintWindow)
  protected
    procedure CreateParams(var Params: TCreateParams); override;
    procedure WMNCPaint(var msg: TMessage); message WM_NCPAINT;
  end;

function IsWinXP: Boolean;
begin
  Result := (Win32Platform = VER_PLATFORM_WIN32_NT) and
    (Win32MajorVersion >= 5) and (Win32MinorVersion >= 1);
end;

procedure TXPHintWindow.CreateParams(var Params: TCreateParams);
const
  CS_DROPSHADOW = $00020000;
begin
  inherited;
  if IsWinXP then
    Params.WindowClass.Style := Params.WindowClass.Style or CS_DROPSHADOW;
end;

procedure TXPHintWindow.WMNCPaint(var msg: TMessage);
var
  R: TRect;
  DC: HDC;
begin
  DC := GetWindowDC(Handle);
  try
    R := Rect(0, 0, Width, Height);
    DrawEdge(DC, R, EDGE_ETCHED, BF_RECT or BF_MONO);
  finally
    ReleaseDC(Handle, DC);
  end;
end;

initialization
  HintWindowClass := TXPHintWindow;
  Application.ShowHint := False;
  Application.ShowHint := True;
end.