Use Knoppix cd (or any other small live linux distri)

boot: linux single

Remove the password from Grub configuration file

You need to mount a partition where you grub configuration file is stored. Depend upon your Linux distribution the location of Grub configuration file changes. Following list summaries Grub configuration file location for pouplar distros:

Debian Linux Grub configuration file location: /boot/grub/menu.lst
Suse Linux Grub configuration file location: /etc/grub.conf
Red Hat/Fedora Linux Grub configuration file location: /boot/grub/grub.conf

If you forgot partition name use the fdisk -l command to obtained partition list / names:

# fdisk -l

Finally, you need to mount your partition as follows:

# mount /dev/sdb1 /mnt

Open Grub configuration file in editor and remove a line like this:

password --md5 $1$9sdflksdf/sdf44k