[TAG] How do I reformat an Ubuntu installation?

[Derry]

I have SUSE 10 on my PC and installed Ubuntu which has partitioned my
hard drive. I want to reformat the hard drive to allow a clean reinstall
of Ubuntu 7.10 only from a desktop CD. I've tried using "fdisk" but get
the error message "Unable to open /dev/hda" which suggests a problem
somewhere, yet Ubuntu currently runs with out any problems. Do I
continue with fdisk? If so, how? Or try something else? Once
reformatting is successful, will the PC boot directly off the CD or do I
need to make a boot disk of some kind? Many thanks for any tips.