Download the version you want from herehttp://forum.notebookreview.com/windows-os-software/604187-legal-download-digitalriver-windows-7-sp1-13-languages.html
Burn it to a disk. Then either clean install it to your laptop by formatting the HDD, or create a partition to install it to. The 2nd option retains any stored data on the drive, you should then remove the old version of windows. You would then need to install all drivers from your laptop manufacturer's website. Windows would need to be re-activated using the key on the base of your laptop.