If you have an old computer that you'd like to sell on, you should be careful about wiping it clean before it leaves your hands. If you just delete files, they aren't really deleted completely, it is possible to recover them if you have the right tools (Kernel is good for this).
To delete files completely special software is needed to write over the places on the hard disk where the files are to ensure the data cannot be recovered. This process has many levels of security. If you want the highest security you'll have to purchase a program. However, for most of us a good level of erasing files on our hard disks is enough.
I've found KillDisk to be easy to use and effective.
I downloaded the Linux Console version. It's very small, it's possible to write it to a small USB flash drive. In my case I had an old 256 MB flash drive that I used. Download the zip, open it and run the ISOtoUSB.exe. Be careful to point it at your USB flash drive!!! It'll write to the drive in seconds.
Insert your USB flash drive into the target computer (the one you want to wipe) and boot the computer. You may have to press a key, maybe F10 or F12 to boot from the USB flash drive. Once it boots up you'll be presented with a blue text screen. There are menus and you can use a mouse. It's relatively easy to use.
You can select any disk on the computer. Be careful which one you select! Use the arrow keys and press Space to select an option.
With the free version you can select a security level where it writes zeros over the data on the disk. This is not 100% security but it's pretty good for home use. If you have some real secrets to hide you could always upgrade to the Pro version!
Before it will start you must type some words - this is to ensure you really want to wipe the computer! Think before you do this! There is no undo after using KillDisk!
Once you start it, it'll take some time to wipe the data. Here's a photo I took of me wiping my PC:
It is quick and easy to use. I like the simplicity. I used this before in the past with the DOS version years ago. This version is just as good. I especially like how small it is.