I don;t think that you can format it while the operating system is running as it needs the C: drive to run.
Can you find out the manufacturer of the hard drive? If you can then I would go to their website and download a small file that can be used to create a boot disc from which you can format the drive. The boot disc can usually be a floppy or a CD but if it is a CD you would need to go into the BIOS setup to make the CD boot before the hard drive.
Seagate hardrives for instance have a section on their website where you can create an installation disc where you can then do such things as format a hard drive without going into the operating system