Obtain a windows 7 disc (Either torrent one and burn it, or borrow one from a friend)
Make sure the OS is the same as you have currently installed ( Like windows 7 home premium etc;)
Insert the DVD, and hit F12, select the DVD drive to be the boot disc, save and restart.
When it boots, and says "Hit any key to boot from CD" strike any key.
Now, it will load the disc, go through the options and find your drives.
Select the drive you want to boot windows onto, then click next.
Select custom install and folow the directions.
You will put your product key located under your laptop in one of the steps in this process.
Once finished after a few reboot's, your all done.
Optional: if you want to get rid of EVERYTHING from the old operation system, go to my computer and find were it says "Windows.old" and delete that file.
Finally, look go to the start menu and search for "activate windows" run the wizard and your finished.