Eraser is an advanced security tool (for Windows), which allows you to completely remove sensitive data from your hard drive by overwriting it several times with carefully selected patterns. The patterns used for overwriting are based on Peter Guttmann's paper 'Secure Deletion of Data from Magnetic and Solid-State Memory' and they are selected to effectively remove magnetic remnants from the hard drive.
More info:
hxxp://www.heidi.ie/eraser/