I went on three weeks holiday recently and when I got back I had forgotten what I had configured as the GRUB and root passwords on a CentOS 5.1 box.

I ended up using two FAQ's that I stored for future reference. One I had written - https://toggen.com.au/faqs/mine/Remove_GRUB_password_and_reset_root_password.html and the other I had gleaned from the Linux Users Mailing list submitted by Kurt Wall - https://toggen.com.au/faqs/others/copyDataFromFC_LVM_to_Ubuntu/copyDataFromFC_LVM_to_Ubuntu.txt

Because the box I was locked out of was built using Logical Volumes (LVM) I had to mount the partitions using the copyDataFromFC_LVM_to_Ubuntu.txt how to.

Removing the GRUB password and resetting root password to blank was covered in Remove_GRUB_password_and_reset_root_password.html

I haven't updated them to be current but if you have a reasonable understanding of Linux you should be able to use them as pointers.