1. Download Keyfinder and run it to recover the Product Key code.
2. Look in Control Panel > System and Security > System... for the Windows 7 version you have installed.
3. Go to the link and download the same Windows 7 version ISO...
4. Burn the ISO to a DVD disk, or to a USB Flash pendrive using Windows 8 USB Installer Maker
5. Restart the computer and press the key as instructed on the BIOS post to access the Boot Menu, select the DVD or USB Flash drive, and use the Windows installer to format the HDD... the option to format will be available after you select the Custom (Advanced) installation on the 4th installer Window.
Keyfinder
http://www.softpedia.com/get/Multimedia/Audio/Other-AUDIO-Tools/IS-KeyFinder.shtml
Windows 7...