Ohh!
mzilikazi
Could you provide a bit more information?
My knoppix doesnt have anything called XFCE4 but this can change res on the fly or how
will that solve my problem?

" install a different desktop just to get different resolutions now did you"

Im not sure what you mean. Another windows manager ?

I just want that my user "A" always runs in 800x600 but root has
1024x768.

Thanks.