In Windows, if its not your OS drive, you can just right click on it and format.
If it is your OS drive, just get a 9x boot floppy (www.bootdisk.com) and boot with that in then its just "format c:" without the quotes.
If its your OS drive and you are wanting to format it to start clean, you can just begin the re-installation of Windows and the setup will ask you if you want to format the drive.