Physically remove (or disconnect the 2 cables attached to) the 64Gb drive.
Start the computer with the Windows 8 CD in the CD/DVD Drive and follow the Windows 8 installation steps.
Note that if your computer is not set to boot from CD first you may have to enter BIOS (when you boot your computer, the first screen you see would normally show the BIOS/Setup key needed to enter BIOS/Setup - usually DELete, ESCape or one of the F keys [F1, F2 etc]) to change the BOOT ORDER (make sure CD/DVD drive is the first in the boot sequence).
Once in the installation setup it will ask you on what drive you want to install Windows 8 to. If you really don't need anything on the 300Gb drive, simply select any partition and hit 'Delete'. Once all partitions are deleted, click 'Next' and the installation process will do the rest on its own (that is, create the 350mb 'System Reserved' partition and the (new) C: partition.
No need for diskpart.