Для того чтобы сделать это выполните следующие шаги:

      Срздайте новый проект. 

      Установите FormStyle формы в fsMDIForm 

      Разместите Image на форме и загрузите в него картинку. 

      Найдите { Private Declarations } в обьявлении формы и добаьте следующие строки: 

                 FClientInstance : TFarProc; 

                 FPrevClientProc : TFarProc; 

                 procedure ClientWndProc(var Message: TMessage); 

      Добаьте следующие строки в разделе implementation: 

             procedure TMainForm.ClientWndProc(var Message: TMessage); 

             var 

               Dc : hDC; 

               Row : Integer; 

               Col : Integer; 

             begin 

               with Message do 

                 case Msg of 

                   WM_ERASEBKGND: 

                   begin 

                     Dc := TWMEraseBkGnd(Message).Dc; 

                     for Row := 0 to ClientHeight div Image1.Picture.Height do 

                       for Col := 0 to ClientWidth div Image1.Picture.Width do 

                         BitBlt(Dc, 

                            Col * Image1.Picture.Width, 

                            Row * Image1.Picture.Height, 

                            Image1.Picture.Width, 

                            Image1.Picture.Height, 

                            Image1.Picture.Bitmap.Canvas.Handle, 

                            0, 

                            0, 

                            SRCCOPY); 

                       Result := 1; 

                   end; 

                   else 

                     Result := CallWindowProc(FPrevClientProc, 

                                              ClientHandle, 

                                              Msg, 

                                              wParam, 

                                              lParam); 

               end; 

             end; 

             В методе формы OnCreate добавьте: 

                FClientInstance := MakeObjectInstance(ClientWndProc); 

                FPrevClientProc := Pointer(GetWindowLong(ClientHandle, 

                                           GWL_WNDPROC)); 

                SetWindowLong(ClientHandle, 

                              GWL_WNDPROC, LongInt(FClientInstance)); 

             Добавьте к проекту новую форму и установите ее свойство FormStyle в 

             fsMDIChild. 

             У Вас получился  MDI-проект с "обоями" в клиентской области MDI формы. 

В примере для глобального перехвата нажатия клавиши printscreen регистрируется горячая клавиша (hot key).
Пример :

            type 

               TForm1 = class(TForm) 

                 procedure FormCreate(Sender: TObject); 

                 procedure FormDestroy(Sender: TObject); 

               private 

                 { Private declarations } 

                 procedure WMHotKey(var Msg : TWMHotKey); message WM_HOTKEY; 

               public 

                 { Public declarations } 

               end; 

             var 

               Form1: TForm1; 

             implementation 

             {$R *.DFM} 

             const id_SnapShot = 101; 

             procedure TForm1.WMHotKey (var Msg : TWMHotKey); 

             begin 

               if Msg.HotKey = id_SnapShot then 

                 ShowMessage('GotIt'); 

             end; 

             procedure TForm1.FormCreate(Sender: TObject); 

             begin 

               RegisterHotKey(Form1.Handle, 

                              id_SnapShot, 

                              0, 

                              VK_SNAPSHOT); 

             end; 

             procedure TForm1.FormDestroy(Sender: TObject); 

             begin 

               UnRegisterHotKey (Form1.Handle, id_SnapShot); 

             end; 

Spoolerа печати Windows посылает WM_SPOOLERSTATUS каждый раз при добавлении и удалении заданий в очереди печати. В следующем примере показано как перехватить это сообщение
Пример :

             type 

               TForm1 = class(TForm) 

                 Label1: TLabel; 

               private 

                 { Private declarations } 

                 procedure WM_SpoolerStatus(var Msg : TWMSPOOLERSTATUS); 

                   message WM_SPOOLERSTATUS; 

               public 

                 { Public declarations } 

               end; 

             var 

               Form1: TForm1; 

             implementation 

             {$R *.DFM} 

             procedure TForm1.WM_SpoolerStatus(var Msg : TWMSPOOLERSTATUS); 

             begin 

               Lable1.Caption := IntToStr(msg.JobsLeft) + 

                                 ' Jobs currenly in spooler'; 

               msg.Result := 0; 

Из реестра. См. пример.
Пример :

             uses Registry; 

             procedure TForm1.Button1Click(Sender: TObject); 

             var 

               reg : TRegistry; 

               ts : TStrings; 

               i : integer; 

             begin 

               reg := TRegistry.Create; 

               reg.RootKey := HKEY_LOCAL_MACHINE; 

               reg.OpenKey('hardware\devicemap\serialcomm', 

                           false); 

               ts := TStringList.Create; 

               reg.GetValueNames(ts); 

               for i := 0 to ts.Count -1 do begin 

                 Memo1.Lines.Add(reg.ReadString(ts.Strings[i])); 

               end; 

               ts.Free; 

               reg.CloseKey; 

               reg.free; 

             end; 

Функция SHELLAPI ExtractIconEx:
Обратите внимание - в примере функции обьявленны иначе, чем в модуле ShellAPI

             type ThIconArray = array[0..0] of hIcon; 

             type PhIconArray = ^ThIconArray; 

             function ExtractIconExA(lpszFile: PAnsiChar; 

                                     nIconIndex: Integer; 

                                     phiconLarge : PhIconArray; 

                                     phiconSmall: PhIconArray; 

                                     nIcons: UINT): UINT; stdcall; 

               external 'shell32.dll' name 'ExtractIconExA'; 

             function ExtractIconExW(lpszFile: PWideChar; 

                                     nIconIndex: Integer; 

                                     phiconLarge: PhIconArray; 

                                     phiconSmall: PhIconArray; 

                                     nIcons: UINT): UINT; stdcall; 

               external 'shell32.dll' name 'ExtractIconExW'; 

             function ExtractIconEx(lpszFile: PAnsiChar; 

                                    nIconIndex: Integer; 

                                    phiconLarge : PhIconArray; 

                                    phiconSmall: PhIconArray; 

                                    nIcons: UINT): UINT; stdcall; 

               external 'shell32.dll' name 'ExtractIconExA'; 

            procedure TForm1.Button1Click(Sender: TObject); 

             var 

                 NumIcons : integer; 

                 pTheLargeIcons : phIconArray; 

                 pTheSmallIcons : phIconArray; 

                 LargeIconWidth : integer; 

                 SmallIconWidth : integer; 

                 SmallIconHeight : integer; 

                 i : integer; 

                 TheIcon : TIcon; 

                 TheBitmap : TBitmap; 

             begin 

               NumIcons := 

               ExtractIconEx('C:\Program Files\Borland\Delphi 3\BIN\delphi32.exe', 

                             -1, 

                             nil, 

                             nil, 

                             0); 

               if NumIcons > 0 then begin 

                 LargeIconWidth := GetSystemMetrics(SM_CXICON); 

                 SmallIconWidth := GetSystemMetrics(SM_CXSMICON); 

                 SmallIconHeight := GetSystemMetrics(SM_CYSMICON); 

                 GetMem(pTheLargeIcons, NumIcons * sizeof(hIcon)); 

                 GetMem(pTheSmallIcons, NumIcons * sizeof(hIcon)); 

                 FillChar(pTheLargeIcons^, NumIcons * sizeof(hIcon), #0); 

                 FillChar(pTheSmallIcons^, NumIcons * sizeof(hIcon), #0); 

                ExtractIconEx('C:\Program Files\Borland\Delphi 3\BIN\delphi32.exe', 

                               0, 

                               pTheLargeIcons, 

                               pTheSmallIcons, 

                               numIcons); 

                {$IFOPT R+} 

                  {$DEFINE CKRANGE} 

                  {$R-} 

                {$ENDIF} 

                 for i := 0 to (NumIcons - 1) do begin 

                   DrawIcon(Form1.Canvas.Handle, 

                            i * LargeIconWidth, 

                            0, 

                            pTheLargeIcons^[i]); 

                   TheIcon := TIcon. Create; 

                   TheBitmap := TBitmap.Create; 

                   TheIcon.Handle := pTheSmallIcons^[i]; 

                   TheBitmap.Width := TheIcon.Width; 

                   TheBitmap.Height := TheIcon.Height; 

                   TheBitmap.Canvas.Draw(0, 0, TheIcon); 

                   TheIcon.Free; 

                   Form1.Canvas.StretchDraw(Rect(i * SmallIconWidth, 

                                                 100, 

                                                 (i + 1) * SmallIconWidth, 

                                                 100 + SmallIconHeight), 

                                            TheBitmap); 

                   TheBitmap.Free; 

                 end; 

                {$IFDEF CKRANGE} 

                  {$UNDEF CKRANGE} 

                  {$R+} 

                {$ENDIF} 

                 FreeMem(pTheLargeIcons, NumIcons * sizeof(hIcon)); 

                 FreeMem(pTheSmallIcons, NumIcons * sizeof(hIcon)); 

               end; 

             end; 

             end.