In Mandriva control center, in the package installation tool, make sure either "task-gnome" or "task-kde" is installed. Alternately, you can also select "task-ede", "task-e17", or "task-xfce". And in addition, you could install "task-3ddesktop", but tyt will be when you have at least one working 2D desktop.
That's for Mandriva 2008.0. Hopefully 2008.1 is not too different.