procedure TMyForm.Button1Click(Sender: TObject);
Var
  KeyState:  TKeyboardState;
begin
  GetKeyboardState(KeyState);
  if (KeyState[VK_NUMLOCK] = 0) then
    KeyState[VK_NUMLOCK] := 1
  else
    KeyState[VK_NUMLOCK] := 0;
  SetKeyboardState(KeyState);
end;