I don't know exactly how stuff works on Mandrake/Mandriva, but some generic ideas would work. I am building my ideas on a Slackware notebook.
1) a- Get a Linux installation CD, or any bootable one, and boot into your system by giving a parameter, for example, "root=/dev/hda3".
b- Boot into a live linux CD and mount your root partition, then chroot there.
In short, boot into your system somehow. It is not that hard, and includes things every linux user should know (IMHO). Read gentoo's handbook anyway, it will help you in the rest of your life.
2) a- If you have lilo, then say "lilo". If grub, then "grub-install /dev/hdx" should work(replace x with what applies to your disk).