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  TaskbarVisibleMode(False);
  DesktopVisibleMode(False);
  ShowWindowsStartButton(False);                    

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  TaskbarVisibleMode(True);
  DesktopVisibleMode(True);
  ShowWindowsStartButton(True);                    

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procedure ShowWindowsStartButton(bvisible: Boolean);
var
  h: hwnd;
  TaskWindow: hwnd;
begin
  if bvisible then
  begin
    h := FindWindowEx(GetDesktopWindow, 0, 'Button', nil);
    if h<>0 then
    begin
      TaskWindow := FindWindow('Shell_TrayWnd', nil);
      ShowWindow(h, 1 );
      Windows.SetParent(h, TaskWindow);
    end;
  end
  else
  begin
    h := FindWindowEx(FindWindow('Shell_TrayWnd', nil), 0, 'Button', nil);
    if h<>0 then
    begin
      ShowWindow(h, 0);
      Windows.SetParent(h, 0);
    end;
  end;
end;                    

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procedure TaskbarVisibleMode(bVisible: boolean);
var
  HTaskbar: HWND;
begin
  HTaskBar := FindWindow('Shell_TrayWnd', nil);
  if HTaskBar<>0 then
  begin
    EnableWindow(HTaskBar, bVisible);
    if bVisible=False then
      ShowWindow(HTaskbar, SW_HIDE)
    else
      ShowWindow(HTaskbar, SW_SHOW);
  end;
end;                    

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procedure DesktopVisibleMode(bVisible: boolean);
var
  HDesktop: HWND;
begin
  HDesktop := FindWindow('progman', nil);

  if HDesktop<>0 then
  begin
    if bVisible=False then
      ShowWindow(HDesktop, SW_HIDE)
    else
      ShowWindow(HDesktop, SW_SHOW);
  end;
end;                    

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  ModifyWorkArea(0,6,0,0); //ein oberer Rand von 6 pixeln entsteht, ähnlich dem, der Taskbar                    

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  SetWorkareaToStandard;                    

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//http://www.delphi-library.de/viewtopic.php?t=18442&highlight=taskbar
procedure SetWorkareaToStandard;
{ set workarea to standard screen (= excluding taskbar) }
var
  hApp: HWND;
  rcApp, rcWork: TRect;
begin
  { prepare a default fullscreen workarea }
  rcWork.Top:=0;
  rcWork.Left:=0;
  rcWork.Bottom:= GetSystemMetrics(SM_CYSCREEN);
  rcWork.Right:= GetSystemMetrics(SM_CXSCREEN);

  { get the taskbar handle }
  hApp := FindWindow('Shell_TrayWnd', '');
  if hApp <> 0 then begin
    { get the size of the taskbar }
    GetWindowRect(hApp, rcApp);
    { cut the workarea to place the taskbar }
    if rcApp.Right<rcWork.Right then
      rcWork.Left:=rcApp.Right; { bar on left edge }
    if rcApp.Bottom<rcWork.Bottom then
      rcWork.Top:=rcApp.Bottom; { bar on top edge }
    if rcApp.Left>0 then
      rcWork.Right:=rcApp.Left; { bar on right edge }
    if rcApp.Top>0 then
      rcWork.Bottom:=rcApp.Top; { bar on bottom edge }
  end;

  { set workarea } 
  SystemParametersInfo (SPI_SETWORKAREA, 0, @rcWork, SPIF_SENDCHANGE );
end; { StandardArea }                    

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procedure ModifyWorkArea(VLeft,VTop,VRight,VBottom:integer);
var
  rcWork: TRect;
begin
  rcWork.Top:=VTop;
  rcWork.Left:=VLeft;
  rcWork.Bottom:=GetSystemMetrics(SM_CYSCREEN)-VBottom;
  rcWork.Right:=GetSystemMetrics(SM_CXSCREEN)-VRight;
  SystemParametersInfo (SPI_SETWORKAREA, 0, @rcWork, SPIF_SENDCHANGE);
end;                    

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uses Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs,ExtCtrls,PsAPI,TLHelp32,ShellAPI,ShlObj,ActiveX;