You can't do this while running the install from the internal disk. It has to be from an external device, such as a USB CD reader. Can you borrow one? The way you'd do this is plug in the CD reader (on the Mini 9, to the USB port closest to the power jack), insert the CD. When the Dell logo appears, press 0 and select to boot from USB. Then choose a Custom (or "Advanced", I forget which) install and then delete the internal disk partitions.
It may also be possible to create a bootable USB flash drive from your CD (on another computer).