bring up a terminal and type
password (whatever the root password is)
now you have full access for what you need.
If you need to run a complete X session as root you will need to add the hidden user root as a login in the gui (goto the menu...configuration...KDE...system...login manager...usertab and uncheck root as being hidden).
You can have X not autostart (goto mandrake control center...boot...drakboot...uncheck the launch the graphical environment when your system starts) and then login as root from the consol and then type startx.
I would recommend that you us the terminal and SU for all root work simply because it is a safer habit to be in while having so much power :-}.