Did you know when you delete a file on your computer it's not really gone, even when you empty the recycle bin? If you going to sell/donate your computer, these files can be recovered simply, and even using free software (see pic below). In this instructable I intend to show you how to save your data from being recovered using free software, and the only thing you might have to buy is a blank cd.
Step 1: Before we start...
There are three main ways you can erase your data:
1) Nuking the hard drive:
Is where you use a cd-rom to remove all traces of ANYTHING on your computer, including Windows. This means that you will not be able to boot into Windows after nuking, so you will have to reinstall windows (unless you specified when selling). This way is probably quickest.
2) Shredding your files:
Is where you haven't yet deleted the file, and you shred this one file instead of deleting it.
3) Erasing Free Space:
Is where you remove traces of things you may have previously deleted. This keeps your operating system.