If you don't have your Windows 7 disc handy—but want to create a custom installation, run Windows from a USB drive, or just do a fresh install—you'll need an ISO file of the disc.
http://lifehacker.com/microsoft-now-lets-you-download-windows-7-isos-with-a-v-1688039211?utm_campaign=socialflow_lifehacker_facebook&utm_source=lifehacker_facebook&utm_medium=socialflow