Затычки мыслей

A very clear article on the topic about Ubuntu is here. I’ve checked it against Debian lenny, it works.
Here are short steps:
1. Enter grub menu, edit kernel line so that it has init=/bin/bash kernel parameter, no ‘single’ parameter should be specified.
2. Boot the kernel, it will stop in bash prompt.
3. Remount / and /proc read-write: mount -o remount,rw /; mount -o remount,rw /proc
4. Run passwd.