If you didn't know, when you delete a file or empty your trash can, it isn't actually wiped from your hard drive.

If you didn't know, when you delete a file or empty your trash can, it isn't actually wiped from your hard drive. Using special tools, deleted files can be recovered by the authorities. That sensitive love letter — or photo, or video — that you think has been safely deleted, can be easily dug up by anyone sitting at your computer. To make sure that a deleted file stays deleted, you must securely wipe your hard drive. Don't worry, it's easier than it sounds.

The first approach to securely deleting files is just to wipe your entire hard drive. This is ideal if you want to sell your computer, or if you're giving it to a friend or family member.

First, a word of warning: If you wipe your hard drive, your computer will be completely unusable. After wiping, you will not be able to boot into Windows or OS X. If you only want to wipe a single file (or a few files), follow the instructions further down the page.

First, download DBAN (it's free). Insert a blank CD into your CD drive. If you're using Windows 7 or 8, right click the file and click "Burn disc image." If you're using Windows XP or Vista, use CDBurnerXP. If you're using Mac OS X, use Disk Utility.

Whichever route you take, you will end up with a CD with DBAN on it. Now restart your computer, and if everything goes to plan your computer will start DBAN instead of Windows or OS X. If your normal operating system loads, restart your computer again but this time, when prompted, tell your computer to boot from the CD-ROM drive. This process varies from computer to computer, so you may need to search to find out how this is done on your system.

Once the DBAN welcome screen appears, hit Enter, then select your hard drive from the list (if you have multiple hard drives be sure to pick the right one!) Both Quick Erase and DoD Short should be more than enough to prevent people from snooping on your data — but if you want to be really certain that no one, including the feds, can read your data, use 8-pass PRNG Stream.

Be aware that, depending on which method you use to wipe your hard drive, the process could take anywhere from a few hours to a few days. Suffice it to say, if you're wiping a laptop hard drive, make sure it's plugged in before you start.

How to securely delete individual files

Secure Delete in Mac OS XWindows users, if you only want to delete a handful of files — sensitive documents or images, perhaps — instead of wiping your whole hard drive, you should download Eraser (it's free). To use Eraser, simply install and run it. Like when you wipe your whole drive, you will need to select a secure delete method (DoD 3-pass is fine). Eraser also has the ability to run a scheduled secure delete, if there's a specific file or folder that you want erased every day/week/month.

If you're a Mac user, your Trash basket actually has a secure delete function built in. Open Finder, then simply click "Secure Empty Trash." You can also make it so that emptying the Trash is always secure, by clicking Preferences > Advanced > "Empty trash securely."

Hit it with a hammer

A hard drive shredderIf you don't have time to completely wipe your hard drive (it can take days), your best option is to physically break it — with a sledge hammer. Whack it a few times until the chassis and the platters inside are broken, and then — ideally — bury the remnants in the garden, or scatter the fragments in a few different trash cans.

If you're a larger institution with a ton of drives to wipe, or you don't have access to a sledge hammer, another option is to find a company that will actually shred your drives using specialized, industrial machines (pictured right). Governmental agencies usually go down this route.

Why delete when you can encrypt?

Finally, a fourth option: Instead of wiping your hard drive, you can simply encrypt it. TrueCrypt is by far the best tool for the job, and it's free.

When your drive is encrypted, it's almost impossible for anyone — including the police — to read the contents of your hard drive. It's important to note, though, that if you log in and then leave your computer unattended, your files would be readable by anyone who happens to sit down at your computer. Wiping your hard drive is definitely the most secure option, but encrypting is a lot easier, and it removes the stress of making sure you always delete sensitive files on time.