[NumLock]

Код:

type
  TForm1 = class(TForm)
  private
    { Private declarations }
    procedure WMSYSCOMMAND(var Msg: TMessage); message WM_SYSCOMMAND;
...
procedure TForm1.WMSYSCOMMAND(var Msg: TMessage);
begin
  if Msg.WParam=SC_MINIMIZE then
  begin
    // do it
  end;
  inherited;
end;