You may need to go to the mandrakelinux website http://www.mandrakelinux.com
for an in depth setup guide. It's usually good to have the root partition (/) and the /home partition seperate so that if you upgrade your system all the files in your home folder are kept intact and I think thats why Mandrake created two partitions.
There are two main methods for installation, the express installation where there are few options and the expert installation which gives you more control of the process. When your computer boots up from the Mandrake 9.x or 10 cd type "F1" then "expert" and you'll be in expert mode. When it comes to partitioning, choose custom and then delete the linux partitions. Create a single linux root (/) partition and fill up the entire disk space if you don't want to leave any free space and continue with the install process.
When you get to the configuration of services and bootup, ensure the choice for the bootloader says "lilo" and not "grub" . You can also choose to install lilo on a floppy as a back up or as the main method of booting into Linux without overwriting your current system settings. It's usually safe to start linux from the hard disk because LILO and GRUB recognise windows and it's included in the boot menu, so you can choose which OS you want to run when your system starts up. If you need more info on LILO and GRUB try using google.