You cannot delete the C: drive while Windows 7 is actively running on it. You must use Method 1 (Bootable Media) or Method 2 (Installation Disc) to wipe the system drive.
Once a drive is deleted/cleaned, data recovery is difficult (or impossible if overwritten). Double-check you have backups before clicking Delete or Clean.
If you are giving the computer away and want to ensure data cannot be recovered, use the "Clean All" command.
Use a bootable USB with GParted or a Windows 7 repair disc:
The most secure way to wipe a drive using Windows 7's own tools is through the utility. This method requires booting from a Windows 7 Installation Disc because you cannot wipe the drive while the operating system is currently running on it. Steps to use Diskpart: