Secure Eraser

Eraser is an advanced security tool, which allows you to
completely remove sensitive data from your hard drive by
overwriting it several times with carefully selected
patterns.

You can drag and drop files and folders to the on-demand
eraser, use the convenient Explorer shell extension or use
the integrated scheduler to program overwriting of unused
disk space or, for example, browser cache files to happen
regularly, at night, during your lunch break, at weekends or
whenever you like.

The patterns used for overwriting are based on Peter
Gutmann's paper "Secure Deletion of Data from Magnetic and
Solid-State Memory" and they are selected to effectively
remove the magnetic remnants from the hard disk making it
impossible to recover the data.

Other methods include the one defined in the National
Industrial Security Program Operating Manual of the US
Department of Defense and overwriting with pseudo-random data
up to one hundred times.