From Hermetic Systems
When Windows 'deletes' a file it does not actually destroy the contents of the file but rather simply unlinks the file from the file directory system, leaving the contents of the file in the disk sectors. This data will remain there until the operating system uses those sectors when writing new data. Until the old data is overwritten (which take months or longer) it can be recovered by programs that read disk sectors directly, such as forensic software (so called because it is used to obtain evidence in criminal investigations and also in legal discovery).
In order to be sure that a deleted file really is deleted, gone for good, its contents never to be seen again, it is necessary to overwrite the data sectors of that file. Data Destroyer purges data in files, where 'purge' means to destroy, to eliminate completely by overwriting, so that the data cannot be recovered by any means. Data Destroyer allows you to destroy the contents of a file by overwriting it several times (or many times) with random bytes, and with bytes with alternating bit patterns, before the file is deleted.
Data Destroyer can purge: (a) a single file, (b) all files in a folder, (c) a specified subset of files in a folder, (d) all files in a folder and in selected subfolders of that folder, (e)