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function LockWS: Boolean;
type
  TLockWorkStation = function: Boolean;
var
  hUser32: HMODULE;
  LockWorkStation: TLockWorkStation;
begin
  // Here we import the function from USER32.DLL
  hUser32 := GetModuleHandle('USER32.DLL');
  if hUser32 <> 0 then
  begin
    @LockWorkStation := GetProcAddress(hUser32, 'LockWorkStation');
    if @LockWorkStation <> nil then
    begin
      LockWorkStation;
      Result := True;
    end;
  end;
end;

Beispiel :

procedure TForm1.Button1Click(Sender: TObject);
begin
   LockWorkStation;
end;