boot from CentOS DVD and type the following command at the installation boot prompt:
You are prompted to answer a few basic questions, including which language to use. It also prompts you to select where a valid rescue image is located.
now u will get a shell after some questions, now at shell type
# chroot /mnt/sysimage
This is useful to chroot if you need to run commands such as rpm that require your root partition to be mounted as /. To exit the chroot environment, type exit to return to the prompt.
Now you can troubleshoot the system, check /var/log/messages about what went wrong and than you can stop the problematic services at boot-time.
hope this will help