Runtime rt = Runtime.getRuntime();
Process proc = rt.exec(str);

 public   static   void  main(String[] args) {
        String dirname = "c:/temp/Директория с пробелами/";
        String filename = "Файл с пробелами.xls";
         try  {
            Runtime rt=Runtime.getRuntime();
            Process pr=rt.exec("c:/Program Files/Microsoft Office/OFFICE11/EXCEL.EXE " + "\"" + dirname + filename + "\"" );
        }
         catch  (Exception e) {
        }
    }

Process pr=rt.exec("EXCEL.EXE " + "\"" + dirname + filename + "\"" );