The quickest and easy way to do it is to reformat it and reinstall the OS. All data will be lost and it will be back to were it was when new. You will need your product key so if its not on the laptop make sure to use a key finder before formatting. You will need it to load your OS. The product ID in the windows activation window on your system page IS NOT THE KEY . The key you need is the oem product key. I use this program to find my windows key Magical Jelly Bean Keyfinder.
Hope this helps