If you have lilo, you can tell it to boot like this
linux init=/bin/sh and you'll boot straight into a root shell instead. This can be protected against by adding 'restricted' to each image section in the lilo config, and setting a password in there as well (plain text, so be sure to chmod 600 your lilo.conf). Note that the linux entry there is just the default name - if you've called your image foobar, then it's foobar init=/bin/sh.
You can also whack a debian boot disk in on any linux distro, even with locked lilo suff, and boot from it but don't start the installation. You can then go to a terminal (alt-2) and mount the root filesystem (nomally /dev/hda1 - mount /dev/hda1 /target), then chroot /target, then just run passwd to reset the root password on any distro.