username/password not accepted

[Richard D. Gill]

Do you remember the password which you gave to
root, the system administrator, when you installed
yellowdog? If so, at the login screen simply use
user-name "root" and password ...
Now you are inside your system, including its
graphical user interface, and can do everything,
including resetting passwords of any other users,
erasing vital system files, etc.
It can be done from the command line, but probably
also from the gui using some tools from kde for
managing users. Google can surely help you find
a plain english explanation.

If you don't know your root password you will have
to start up in so-called single user mode, booting
from a rescue disk (your installation cd). Again,
google should help you find a plain english explanation
and amplification of these words.