Choose Grub in the Mandriva Boot Mananger

Lilo is the Boot Manager by default in Mandriva. But you can easily choose grub at Installation time in the Summary phase of the installation.

If you want to choose Grub later on, go in the Mandriva Control Center (Configure your computer) > Boot > Configure the Boot Behavior and choose Grub (graphical or not) instead of Lilo in the first field, then proceed though the end of the wizard without changing anything else and you Boot Manager will be Grub the next time you boot your machine.

You can reverse to Lilo the same way.

Please read Why choosing one of them over the other one before choosing one.