function GetWindText(AHandle: THandle): String;
var
  cb : DWord;
begin
  cb := SendMessage(AHandle, WM_GETTEXTLENGTH, 0, 0);
  SetLength(Result, cb);
  if cb > 0 then
    SendMessage(AHandle, WM_GETTEXT, cb+1, LParam(@Result[1]));
end;

procedure TForm1.Button1Click(Sender: TObject);
var
  AHandle: THandle;
begin
  AHandle := FindWindow('Notepad', nil);
  Win32Check(AHandle <> 0);
  AHandle := FindWindowEx(AHandle, 0, 'Edit', nil);
  Win32Check(AHandle <> 0);
  Memo1.Text := GetWindText(AHandle);
end;