How to give away old computers without risking identity theft from the personal id info on your hard drive?

We just upgraded our computers and were about to donate the old computers to Salvation Army when our neighbor advised us to destroy the hard drives and hence the files in the hard drive before we do so. Seems to be such a waste of still very good computers. Any way out of this? Is there a software that will delete... show more We just upgraded our computers and were about to donate the old computers to Salvation Army when our neighbor advised us to destroy the hard drives and hence the files in the hard drive before we do so. Seems to be such a waste of still very good computers. Any way out of this? Is there a software that will delete personal and other files in the hard drive?
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