You actually need more than a "format c:" equivalent - you need to zap the loader code in the MBR as well in all likelihood.
I do these sort of things from a (Linux) liveCD. Just delete the partitions and try this from a console/terminal
dd if=/dev/zero of=/dev/hda count=200
Adjust the target as necessary. 446 bytes are reserved for the loader code, but it has some flags as well - 200 should be sufficient.
*ALL* your data will be gone - the disk will only be useful for a completely fresh install.