Well, I've never worked with CentOS in particular, but many linux distributions set up a second boot option labelled recovery that logs you in single user mode as root. If you see something like this when booting up try booting into it, then typing the following
passwd <yourusername> (example: passwd bob)
 
It will prompt for the new password, then do it again.
 
After this reboot the system in normal mode.

Here is a mini How To with screenshots about how to recover your password (root) under Fedora (works with CentOS)
 
digg.com/linux_unix/Root_Password_Recovery_Fedora_CentOS
 

You can try ERD Commander 2005.
http://www.pcworld.com/downloads/file_description/0,fid,25445,00.asp
 

This is an old post (3/06), so the person is no longer likely around.
 
Also, the ERD disk is no longer easily available, as Microsoft aquired Winternals, the developer of this utility. All links default to Microsoft's web page.

try this is secret
 
Well, there is yet another way in which we can get root. In the first method, we typed ‘linuxconf’ in the bash shell prompt, however, we could type the following to create a new account with root privileges and without any password:
 
 
 
echo “ankit::0:0:::” >> /etc/passwd
 
 
 
This command will basically edit the /etc/passwd file which is the password file which stores the Passwords and Usernames of all accounts on the machine. One thing to remember here is that you can edit the /etc/passwd file only if you are logged in as root, however, in this case we are not logged in as root, but we have booted into linux single which gives us the root shell. Hence, we can still edit it.
 
 
 
Anyway, to understand how exactly the above command works and how it is able to create a new account without a password, we need to learn the /etc/passwd file is structured.