I just encountered the Password and Parental Lock problem, perhaps after a recent service update by TIVO to enable Yahoo and other internet based services. On my Tivo brand 40 hour DVR, there was no password at all. I know I had set a password before. On our Toshiba brand DVD DVR the password was set to something not known by anyone in our household. We saw the post about entering the default 0000 or 9999. Ours was 8888 found by trial and error.
Good luck finding your default password. When calling TIVO customer support, the computer voice prompter disconnects the call when you identify your TIVO being made by another company.