Just to add some things I think might be worth mentioning... Windows 7 creates a separate 100MB system partition to store the WinRE environment for startup repair, bitlocker, and as a EFI system partition (ESP). By default a volume letter is not assigned, so it doesn't appear in Computer, but it does in Disk Management and Windows Setup partitioning wizard. You can erase a drive including the bootsectors using diskpart which is accessible from Windows Setup using the Shift+F10 shortcut.