Here is a free tool that will completely and securely (use at your own risk) erase your hard disk. No data will be recoverable. Do not use this if you wish to keep the operating system on the machine or do not have the media to reinstall the operating system!
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I should say that it is possible to only erase individual folders with this tool also if you only wish to selectively delete items. My standard practice is to wipe an entire hard disk, reinstall the OS then pass it along.