You can download a free legal ISO image of Windows 7 SP1 from the links below. You will need to download the same version of Windows 7 that you have installed. These images are hosted by the Digital River store which is an official distribution partner of Microsoft.
How to burn ISO image using Windows Burn Disk Image.
Notice: This applies only to Windows 7 and Windows 8, earlier versions do not have this.
1. Place a blank CD or DVD in the tray of your optical drive and close the tray.
2. After you have downloaded the ISO image you want to burn right click on the Start orb, then choose Windows Explorer.
3. When Explorer opens click on Downloads in the left pane. Scroll down till you find the ISO file you want and double click on it. Click on Burn Disk Image.
4. In the image below you will see Disk burner:, this should be set to the optical drive you want to use. Click on Verify disc after burning if you want to Windows to verity the disc image after burn. Click on burn.
5. In the image below you can see that the green progress bar, when the image is finished burning the bar will be filled.
6. After the image has been created click on Close.
Please note: In order to boot from this DVD you may need to change the boot order in the BIOS so that the CD/DVD-ROM is the first device in the boot order, and the hdd is the second device.