I've only installed RedHat but I believe Mandrake is pretty similar. It's very unlikely that Mandrake will just overwrite your 45GB partition, if it's anything like RedHat it'll ask you on which partition to install it. Linux doesn't use FAT32 but Mandrake's installation should be able to format to create the correct filesystem for you. Incidentally, linux actually needs 3 partitions - boot, root and swap. Don't worry though, point it in the direction of that 15GB partition and it should be able to do it all for you.
Click carefully and it'll all be okay