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Editorial Team
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Asked: 2026-05-21T21:52:11+00:00

MY application has had a mode for years where the customer can ‘disable access

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MY application has had a mode for years where the customer can ‘disable access to the OS’. Obviously this feature goes against the grain (at least as far as Windows is concerned) but there are installations where my App is the only program that should ever be visibile to a machine operator amd in this case such a feature is useful.

The technigue I used was built from several ‘layers’:

  1. Hide the taskbar and button.
  2. Disable task-switching.
  3. Disable my main form system icons.

To disable the taskbar I used:

// Get a handle to the taskbar and its button..
Taskbar := FindWindow('Shell_TrayWnd', Nil);
StartButton := FindWindow('Button', Nil);

// Hide the taskbar and button
if Taskbar <> 0 then
  ShowWindow( Taskbar, SW_HIDE );
if StartButton <> 0 then
  ShowWindow( StartButton, SW_HIDE );

// Set the work area to the whole screen
R := Rect( 0,0,Screen.Width,Screen.Height );
SystemParametersInfo(
  SPI_SETWORKAREA,
  0,
  @R,
  0 );

This worked well and still seems fine on W7.
Researching how to disable task-switching some years ago turned up the only technique of ‘pretending’ that your App is a screen saver (other than terrible things like renaming your app to ‘explorer.exe’ and booting into it etc):

procedure EnableTaskSwitching( AState : boolean );
// Enables / disables task switching
begin
  SystemParametersInfo(
    SPI_SCREENSAVERRUNNING,
    Cardinal( not AState),
    nil,
    0 );
end;

Not surprisingly this seems to have no effect in W7 (I think it works in XP etc).
Does anyone know of another, better, way of enabling / disabling Alt-Tab (and other special windows keys) from working?

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1 Answer

  1. Editorial Team

    If found a solution:

    function LowLevelKeyboardProc(nCode: integer; wParam: WPARAM; lParam: LPARAM):
  LRESULT; stdcall;
type
  PKBDLLHOOKSTRUCT = ^TKBDLLHOOKSTRUCT;
  TKBDLLHOOKSTRUCT = record
    vkCode: cardinal;
    scanCode: cardinal;
    flags: cardinal;
    time: cardinal;
    dwExtraInfo: Cardinal;
  end;

  PKeyboardLowLevelHookStruct = ^TKeyboardLowLevelHookStruct;
  TKeyboardLowLevelHookStruct = TKBDLLHOOKSTRUCT;
const
  LLKHF_ALTDOWN = $20;
var
  hs: PKeyboardLowLevelHookStruct;
  ctrlDown: boolean;
begin

  if nCode = HC_ACTION then
  begin

    hs := PKeyboardLowLevelHookStruct(lParam);
    ctrlDown := GetAsyncKeyState(VK_CONTROL) and $8000 <> 0;
    if (hs^.vkCode = VK_ESCAPE) and ctrlDown then
      Exit(1);
    if (hs^.vkCode = VK_TAB) and ((hs^.flags and LLKHF_ALTDOWN) <> 0) then
      Exit(1);
    if (hs^.vkCode = VK_ESCAPE) and ((hs^.flags and LLKHF_ALTDOWN) <> 0) then
      Exit(1);
    if (hs^.vkCode = VK_LWIN) or (hs^.vkCode = VK_RWIN) then
      Exit(1);

  end;

  result := CallNextHookEx(0, nCode, wParam, lParam);

end;

procedure TForm1.FormShow(Sender: TObject);
begin
  SetWindowsHookEx(WH_KEYBOARD_LL, @LowLevelKeyboardProc, 0, 0);
end;

    This disables (as you can see!)

    • Ctrl+Esc (show start menu)
    • Alt+Tab (task switch)
    • Alt+Esc (task switch)
    • Win (show start menu)
    • Win+Tab (3D task switch)
    • Win+D, Win+M, Win+Space, Win+Arrows, Win+P, Win+U, Win+E, Win+F, Win+Digit, …
    • Almost any combination including the Windows key (but not all, e.g. Win+L)
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