^ Nothing will happen, you'll just not have any OS on the machine.
But having Windows on C: shouldn't cause any problem.
Have you yet tried to boot from the Mandriva One LiveCD ?
PS. You can also download and burn Main Page - SystemRescueCd , (SystemRescueCd-x86-1.0.4 (197 MiB)) a LiveCD based on Gentoo. After the text boot sequence just type "startx" at prompt. If everything works you'll find the Gparted graphical partition and format utility among many others.