You can log in using single-user mode and create a new root password.
To enter single-user mode, reboot your computer. If you use the default boot loader, GRUB, you can enter single user mode by performing the following:
At the boot loader menu, use the arrow keys to highlight the installation you want to edit and type [A] to enter into append mode.
You are presented with a prompt that looks similar to the following:
grub append> ro root=LABEL=/
Press the Spacebar once to add a blank space, then add the word single to tell GRUB to boot into single-user Linux mode. The result should look like the following:
ro root=LABEL=/ single
Press [Enter] and GRUB will boot single-user Linux mode. After it finishes loading, you will be presented with a shell prompt similar to the following:
sh-2.05b#
You can now change the root password by typing
passwd root
You will be asked to re-type the password for verification. Once you are finished, the password will be changed. You can then reboot by typing reboot at the prompt; then you can log in to root as you normally would.
Reference: http://www.redhat.com/docs/manuals/enterprise/RHEL-4-Manual/step-guide/s1-q-and-a-root-passwd.html
