Most versions of microsoft windows won't let you format c: while you're running windows because that's the drive that windows is installed on. If it let you do that, as soon as the format started your computer would crash (almost immediately) and everything on the c: drive including your entire windows installation would be completely gone.
If you REALLY want to completely wipe your hard drive, one easy way is to create a bootable floppy disk ("format a: /s" or use the format tool under drive properties and select the "copy system files" option) and then copy from c:\windows\command the utilities fdisk and format. Boot from the floppy disk and run the utility "fdisk". In fdisk, you will need to first remove any partitions listed, then create whatever partitions you want. I usually just create one partition of the maximum size. You should probably set the primary partion as the "active" partition, but some computers will automatically do this for you if no partition is set as active when the computer boots up. Then quit fdisk. The computer will probably say you should reboot, so reboot.
To immediately reinstall windows, remove the floppy and insert the windows installation CD, and reboot. The computer should boot from the cdrom drive and let you install windows. If it doesn't boot from the cdrom drive, you may need to enter your computer's BIOS setup menu (usually by hitting the delete key while the computer is starting up) and in the boot options menu, selecting the cdrom drive as a boot device. Then restart and it should boot up from the cdrom drive.
If you just want to have a blank but bootable hard drive, then immediately after running fdisk and rebooting, run the utility format ("format c: /s"), which will format the c: drive and make it bootable.
To reinstall windows at that point, remove the floppy and put the windows CD in the cdrom drive and reboot. It will hopefully boot using the cdrom drive.
If your computer won't boot froma windows CD in the cdrom drive, you're probably fuxored because accessing the cdrom drive from a dos command prompt requires 2 drivers to be loaded. One driver goes in your config.sys file and is supplied by the cdrom drive manufacturer. The other driver is "mscdex.exe". It goes in the autoexec.bat file and is supplied by microsoft. Explaining how to configure this to work would take a few pages of text, so if you have to do it and don't know how, you may be better off taking your computer to a repair shop and asking for help.