Use this..
http://www.tolvanen.com/eraser/download.shtml
.. to wipe all the free space and file space of the drive using at least the DoD standard 7x random data overwrite. Format, reinstall the operating system, and then do the same overwrite again.
A better way would be to mount the drive as a slave on another PC or laptop, overwrite, format, partition, and overwrite again.