Can't get access root after kernel upgrade

[pravin]

Hi all,

I just installed kernel 2.6.17.6 stable (Knoppix 5.0.1 from CD version).
But everytime I want to launch an application with root access (example KUser, QTParted), when I enter root password, I have the error (bad translation from french):
Could'nt find program su
Verify that variable PATH is defined.

When I launch Konsole, I had the following fatal error:
Could'nt open a PTY...

In summary, I can't access with root!

Thanks for your help.

Pravin