...

function EnumWinProc(Wnd : HWND; frm : TForm1) : Boolean; 
Export; {$IFDEF Win32} StdCall; {$ENDIF}

var
  WinText : Array[0..255] of Char;
begin
  GetWindowText(Wnd, WinText, 255);
  Result := True;
  if (StrPas(WinText) <> '') then
  frm.ListBox1.Items.Add(StrPas(WinText));
end;


procedure TForm1.Button1Click(Sender: TObject);
begin
  EnumWindows(@EnumWinProc, LongInt(Self));
end;

...