When installing Linux, it is important to make sure that you install LILO or GRUB in the root directory or superblock. Do not install LILO or GRUB to the MBR. These programs are used to load the Linux-specific OS kernel (most other OS's include built-in support to load its kernel).
To get the option to install to "First Sector of Boot Partition" in Red Hat 6.1/7 you'll need to have the "Custom" option indented under "install". If you don't, it will install to the MBR. For Mandrake, choose expert install and when it asks where to install the boot loader, change it from the /hda entry (which will install to the MBR) to the correct /hdxx entry (which specifies a partition and hard drive).
