1. Make Sure You have the windows 7 (as well as other products’s license number if any) license number. If your license is not very clear, you need to estimate how many different combination you need to try. You may also use a software to extract the license number, you can use a keyfinder program such as Magical Jelly Bean Keyfinder to pull your product key from the Registry, or from any directory even though your computer did not actually booted it from.
2. Download the windows 7 ISO (you need to match your version) file from Digital River, a licensed distributor of Microsoft products. These downloads are each > 3GB in size, make sure your DVD or USB’s size is larger than this
3. Download Microsoft’s tool: Windows 7 USB/DVD download tool. This utility lets you use the iso file you downloaded in step 2 to burn a bootable DVD or USB flash drive. Whether you choose DVD or USB makes no difference; just confirm that your PC can boot to the media type you select.
4. Boot your computer using the Windows 7 DVD or USB flash drive (you may have to press F12 to choose the media). Follow the prompts to prepare the destination hard drive and install Windows.