Unable to format C drive

I have a dell Inspiron 6000 laptop with windows xp mediacentre edition.I could not format the C drive When i tried to reinstall the Windows XP in "C" drive. C drive dint allow to format C drive and then I reinstalled the OS in "D" drive. Afterwords also , when i tried to format C drive, it is not allowing. What can I do to format the C drive?What would be the reason behind this problem?Please help.