If your problem is to get all of users' data before to potentially format HD, the simplest way is to unscrew your HD and to connect it as slave on another machine. And then backup your data from there.
If your problem is that you don’t have access to the system because you don’t know any user password, then yes there’s a way.
You can log in as root easily if you have a physical access to the machine (using some special key at boot). Once you’re logged as root, you can change password for an existing user, or just add some new user with your own password. Easy.