the bootable installer, when at the point of you telling it where to install WIndows a few steps into the pre-install process, will give you the option of selecting assorted partitions for deletion. (previous MBR installations will normally show 3 created partitions) GIven only one large non-allocated space, the system will automatically do a quick format and start over.
(I've not actually tried deleting/formatting C: partitions from within the active WIndows installation, but, I'd assume it would not allow that)