Оформил: DeeCo
Автор: http://www.swissdelphicenter.ch

  private
     { Private declarations }
     procedure WMSyscommand(var msg: TWmSysCommand); message WM_SYSCOMMAND;
     procedure WMSize( Var msg: TWMSize ); Message WM_SIZE;
   end;

 var
   Form1: TForm1;

 implementation

 {$R *.dfm}

 procedure TForm1.WMSyscommand(var msg: TWmSysCommand);
 begin
    case (msg.CmdType and $FFF0) of
      SC_MINIMIZE: begin
                     ShowMessage('Window about to MINIMIZE');
                   end;
      SC_RESTORE : begin
                     ShowMessage('Window about to RESTORE');
                   end;
      SC_MAXIMIZE: begin
                     ShowMessage('Window about to MAXIMIZE');
                   end;
    end;
    inherited;
 end;

 procedure TForm1.WMSize(var msg: TWMSize);
 begin
   If msg.Sizetype = SIZE_MAXIMIZED then
     ShowMessage('Window MAXIMIZED');
   inherited;
 end;