Erro ao abrir arquivo com Runtime

File rel = new File("c:\\relTeste.xls");
       rel.createNewFile();
       FileWriter relExcel = new FileWriter(rel);
       relExcel.write(new String(sb));

    String sOs = System.getProperty("os.name").toUpperCase();
    if(sOs.equals("WINDOWS NT") || sOs.equals("WINDOWS 2000") || sOs.equals("WINDOWS XP")){
         Runtime.getRuntime().exec(rel.getAbsolutePath());
    } else{
    Runtime.getRuntime().exec("C:\\Program Files\\Microsoft Office\\Office12\\EXCEL.EXE" + rel.getAbsolutePath());
    }

       relExcel.flush();
       relExcel.close();


        }catch (Exception ex){
            ex.printStackTrace();
        }