using command prompt while running the operating system you want to delete won't work
Like I said, or boot with a linux live cd and use cfdisk or something or use a windows98 boot floppy and use format.
To get to know how to use it type /? as a command, then you will get the explanation.
ex: format /?
but if you want to format your c drive an easy way is just:
format C: /Q
That will format your C drive...