Locked out! How to circumvent password urgently?

[Anthony Campbell <ac@acampbell.org.uk>]

I can't access my laptop because neither the user nor the root passwords
work. I can only think I was hacked last night.

I can't access via single user either.

I can get into the system via knoppix. Are there any files in /etc I can
delete in order to make the system accessible without a password? 

This is REALLY urgent because I must use the machine tomorrow, so if all
else fails I shall have to reinstall the system.

Anthony

Using Linux GNU/Debian - http://www.acampbell.org.uk