I am selling my computer and I want to make sure that everything is wiped clean before I sell it. How can I totally delete everything to what it was when I bought it? Do I need to buy something or is there a free program that will delete everything? I just want to make sure that nobody can get any of my personal info from it.

how to clean up files, etc… that may contain personal info, so that i am sending them a 'clean' computer that only has company info on it! i am not the most computer savvy person, so i need this explained to me in step by step directions as if you were explaining it to a 2nd grader. thanks!