You can format your hard drive two ways.
If you just want a clean install of windows, then insert your operating system install disc into the CD drive and restart the machine. Windows will warn you that all will be lost if you proceed (it will format before installing).
If you just want to format the hard drive full stop, then you need a boot disc, if you hav'nt got one you can download one from
www.bootdisk.com.
Restart your machine with the bootdisk in the floppy drive.
When it loads, type C:/ and press enter.
Now type format C and confirm that that is what you want to do.