There is no need for live cd or usb stick.
You could boot into single user mode, mount the filesystems
mount -a and set/change the root password
Single user mode is a mode for just one user, root. You can boot into single user mode without typing in the root password.
In BSD booting into single user mode is usually an option in the boot menu.
In linux booting into single user mode could be achieved by setting
single as a kernel option. (e.g. in grub, select the boot line, press
e to edit, change the init parameter, press
c to continue booting.)
In LILO using the LILO boot prompt (press [Ctrl]-[x] if you're in a graphical interface) and type
If this doesn't work for an old kernel, you can also specify
--init=/bin/bash then you get a root bash shell.