The factory disk of... windows 8? No, you can't install windows 7 with a windows 8 disk.
Also, your computer has the product key on a sticker somewhere on it; at least, it's supposed to.
Sorry, I didn't include enough info. My computer came with windows 7 installed, I put W8 on it and now I want to restore it back to W7 as my friend does not want to buy a computer with W8.
I'll have a search for a product key somewhere, but my PC is 3 years old and if it's on a separate sheet, I've probably lost it. I only have the manual.