Recently I switched to PeaZip from 7zip due to some of the extra features found in PeaZip. One of these was it's ability to securely wipe drives that you wish to sell/get rid of. Today I stumbled across a built in windows utility called cipher that allows you to perform a similar operation. To perform a drive wipe simply enter the following at a command prompt, replacing X for the drive letter of the drive whose freespace you'd like to wipe:
To rapidly wipe a drive first perform a quick format then run cipher.
Here is the /help entry for the /w option:
/W Removes data from available unused disk space on the entire
volume. If this option is chosen, all other options are ignored.
The directory specified can be anywhere in a local volume. If it
is a mount point or points to a directory in another volume, the
data on that volume will be removed.
Wiping a 1TB 5400RPM hard drive with cipher took approximately 6 hours