Some systems require the root password to boot into 'single' mode. In this case, try editing the grub boot line to add 'init=/bin/bash' to the end of the kernel line. This will boot you into a very basic system, using the bash shell instead of init. You may have to mount the root partition read/write:
mount -no remount,rw /
Then use passwd to reset the password.
If grub is protected, try booting from a LiveCD or USB stick. Open a root shell, and use fdisk -l to show the available disk partitions. Mount the root partition with
mount -o,rw /dev/hda1 /mnt
Check it's the right directory with ls /mnt, then change into that as your root directory:
Now use passwd as before.