Step 1: Reset the box
Ctrl-Alt-Delete
Step 2: At the GRUB menu press 'e' to edit the line for the kernel you want to boot
e
Step 3: Append the following to the end of kernel line (to force single user mode without login)
single init=/bin/bash
Step 4: Boot this line
Ctrl-x
Step 5: Remount the filesystem read-write
mount -rw -o remount /
Step 6: Reset the password
passwd
Step 7: Force sync
sync
sync
sync
Step 8: Reboot back into the usual runlevel
reboot