Cannot start centos6.4

after double system reinstalling windows7 Dual system reinstall windows7 can not start centos6.4
1. Download CentOS-6.4 installation CD ISO
2. Use UltraISO to burn it to U disk
3. Restart to enter the BIOS interface, choose to boot from the U disk
4. When you install the CD-ROM interface, select Rescue install system to enter the rescue mode
5. After a series of settings, enter the shell mode
6. Execute the following command to reinstall grub
[php]
# chroot /mnt/sysimage
# /sbin/grub-install /dev/sda
7.View/Edit grub configuration file
[php]
#vi /boot/grub/grub.conf
8. If you don't have a Windows startup item, you can manually add
#Windows
title windows7
root (hd0,0)< Br>makeactive
chainloader +1
9.Save, exit, restart
[php]
#exit
#reboot