If grub allows you to type your choice simply type "linux single" this will drop you into init(1)
Then its as easy as typing passwd root.

On most distros you can also boot off the first cd used for installtion, and type something like
boot: bare.i root=/dev/hda1 noinitrd ro

At least in slack you can. and I know Red-Hat has an option similiar.

Otherwise go with the knoppix route, Or perhaps sysresccd.org tools.