Just back up all of the data that you do need, and get into a safemode command prompt at boot and type:
(of course): format C:
if you have it formated in NTFS though:
(of course): fdisk
and use the utility to empty the HD.
there are many ways of going about this. But if you want to keep the OS without formatting, us _X3_'s method: DATASCRUBBER.