I’m surprised no-one has asked the basic questions:
Is the hard disk you want to format your system disk (ie the one you boot from, the one that contains your operating system files)? If not, then you can format it by opening Windows Explorer, right-clicking it and selecting “Format”. If it is the system disk then you need to know the answer to question 2…
What operating system are you using? If it is Windows 9x, then creating a boot disk is the way to go. You can create this by sticking a floppy disk in the drive, going to a command prompt (click Start, run, then type in “command” and hit enter). At the command prompt type “format a: /s” (don’t type the inverted commas in any of these commands). Once done, make sure you copy the format.exe file to the floppy as you’ll need it when you format the hard disk.
Reboot the system from the floppy disk and do “format c: /s”. This should suffice. From there you’ll need to reinstall the operating system from the installation CD. This might require you to load DOS drivers on the floppy disk. You can find boot disks here http://www.bootdisk.com/bootdisk.htm
If it’s WinNT/Win 2000/Win XP, you’re best off throwing the install CD into the CDROM and booting from it. Tell it you want to reinstall the OS. During this process, delete the existing partition and create a new one - this in itself will force a format.
If you have more than one hard disk in your system, or more than one partition on your disk, make sure you perform these operations on the correct disk!
Disclaimer 1: Please make sure you back up ALL your files before doing any of this. These procedures WILL delete EVERYTHING on the target disk, including operating system files and files required for your system to operate. MAKE SURE you have ALL the installation and driver disks, including drivers for your video card, modem, network card, mouse etc. BEFORE you proceed.
Disclaimer 2: If you balls this up, it is NOT my fault. I’ve given you general guidance, but cannot predict the exact configuration of your PC. If you’re not sure what you’re doing, get someone knowledgable to help you.
Disclaimer 3: I did not intend for Disclaimer 1 or Disclaimer 2 to sound patronising or insult your intelligence