For everybody that need to show something only on the useful area of the desktop I wrote this function:

function DesktopClientArea: TRect;
var
  rDesktop, rWinBar: TRect;
begin
  Windows.GetWindowRect(GetDesktopWindow, rDesktop);
  Windows.GetWindowRect(FindWindow('Shell_TrayWnd', nil), rWinBar);
  SubtractRect(Result, rDesktop, rWinBar);
end;

Hugs!