You can boot off a rescue floppy (like tom's root and boot disk http://www.toms.net/rb/
) or a live CD (like knoppix http://www.knoppix.org/)
, mount your root partition and edit /etc/passwd. Look for the entry for root, remove the "x" from that line,
Now save the file, reboot and now root will not have a password. Don't forget to assign a password for root straight away with the passwd command.