How do you reformat a computer with windows 7?

i tried googleing and doing what i do on my windows XP but i says i cant do anything on it because its in use so i have to do it by cd but i dont have a cd of my windows 7 so is theri any other way to do this or to put it back to factory setting or saving it to a cd i once saw their was a website that could do thi but i forgot what it was called.