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,
eg:
root:x:0:0::/root:/bin/bash
becomes
root::0:0::/root:/bin/bash
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.