How should i format my hard disk?

PLz guide me the steps how to do it via MS-DOS mode... When i am tryin to format my C drive ... it is replyin sumthin like allocation size.... How to solve that problem, format the hard disk, and install a new copy of windows xp.... Please guide me each n every step i will be very thank ful 2 u... N 1 more thing if i want to install directly it is sayin sumthin third party(kernel).. plz solve any 1 of my prob....