How do I format all partitions on the only disk, when only Win 8 Pro is installed?

  1. Prepare a bootable Windows medium. If you don't have a Windows DVD or flash drive, you can download Windows ISO and create one.

  2. Boot using the medium you have just created.

  3. Wait for a window with an Install now button. When it shows up, press Shift+F10. Command prompt will appear.

  4. Type diskpart, press Enter and wait until Diskpart is ready.

  5. Type list disk Enter. A numbered list of disks will be printed. Note the number next to the disk you want to erase.

  6. Type select disk <number> Enter (replace <number> with your disk's number).

  7. Type one of these commands:

    • clean - to quickly erase all partitions without wiping entire disk, data will still be recoverable if one attempts to
    • clean all - to wipe entire hard disk, it will take a long time but data won't be recoverable

Performing full erase is not recommended on SSD disks, which have their lifespan limited by number of write commands. Overwriting entire disk with zeros (using any method or tool, not just clean all) will reduce it significantly. For self-encrypting SSDs resetting the encryption key is way quicker and just as secure, but doesn't wear out SSD cells. For other SSDs ATA secure erase may be a better choice, although performing it is more complex.

GParted Live

  1. Download GParted Live
  2. Burn to CD or USB with Rufus
  3. Boot from CD/USB (in the rare case it won't start run GParted in compatibility mode and it will for sure.)
  4. Delete your partitions.

It's really that easy easy. (As an alternative any popular live linux distro has GParted included.)

If you want to be really sure there's nothing left behind, look at a secure disk erase program. There are several included with UBCD, which you can download and make bootable using the same techniques promoted by other answers here.

But the main point everyone agrees on is, "You can't do this from within Windows."

Whatever happens, you'll need to prepare and boot to a different operating system in order to complete this task.