When selling or returning a laptop you need to wipe the hard drive. Wiping is different than deleting, as deleting leaves traces that can easily be retrieved. Wiping a hard drive will make sure you do not sell or return a laptop with personal information on it. The best way to wipe a laptop is to use a method developed for the Department of Defense. That method writes several passes of random data on your hard drive to make personal data retrieval impossible.
Back up any data you want to keep on another hard drive or on DVDs. You will not be able to get any data back after wiping the hard drive.
If you need to wipe the hard drive on an ongoing basis, purchase a commercial solution that will allow you to automate the task. If you'll need to use that software only once, download a free solution such as DBAN from http://www.dban.org/.
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Download the software to your hard drive. If you are downloading DBAN, the file is compressed into a ZIP (compression method) file. Right click on the file and select "Extract" to extract the file. Use the "burn image" option and select the name-of-the-program.ISO file to burn a CD or DVD. Once the burning process is finished, reboot your computer.
If you bought commercial software, follow the instructions in your manual. If you are using DBAN, type "autonuke" and press "Enter" to start the wiping process.