Click on Custom Install, and then in the drives section click on delete to clean off the partitions, and once the disk is nothing but free space, select that drive. If you are unable to see that stuff, get your hands on a bootable linux distro, boot off the CD, and run gparted to clean off the disk partitions. Or, you can put that drive into a windows machine, launch an administrative command prompt, and clean the disk off with diskpart utility.