If you don`t even know how to format your PC then you are even less likely to be able to re-install Windows along with all the necessary drivers for the motherboard etc.
If you are determined to proceed then you will need a floppy boot disk to use fdisk to re-partition the HDD and format it from DOS for Win.95, 98 or ME.
If you are on XP, reinstalling XP using the original install disk and selecting a fresh install will normally give the option of formatting the HDD first.