You want to discard your old computer and want to securely erase the data from your hard drive. what can you use to do this and what is the process called

This process is called "data erasure" (or "data wiping").

To understand the need for this kind of process, we first need to understand the problem. When you delete a file on your computer, the data is not actually lost. Instead, a pointer to the information is removed. Therefore, your operating system normally no longer "sees" the file, but special tools can still access and read the original information.

This is a problem, if you want to delete data in such a way that it is no longer recoverable by anyone. The exact process differs between various programs, but the idea is always the same: overwrite the parts of the hard drive that stored the original data with garbage. One common algorithm is a variation of overwriting all the relevant bits with "0", followed by a second-pass with "1", and lastly a third-pass with random "0" or "1".

One popular GPL program for Windows and Linux is called "BleachBit". On Mac OS X, there is a built-in tool called "Disk Utility" that offers this functionality.

If you are interested in deleting the *entire* hard drive, the best way is to use the disk formatting tool of the operating system with "Secure Formatting" (or "Safe Formatting") enabled. The number of "passes" is often configurable, but remember that each pass is re-writing your entire hard drive with garbage data, so this process may take a while to finish.