For some odd reason Microsoft has made it quite the task to procure a legitimate ISO image of Windows 7. You would think they would have an easy to find place on their website for ISO images, because remember you really cannot take advantage of these ISO images unless you have a proper legal key. So many people end up losing the disc or ISO they previously had making it quite the challenge to get a replacement.
Microsoft has added a new feature on their website that allows you to get a replacement ISO. It is however behind a bit of protection, you will need to provide your legal product code in order to get to the download page. If you happen to have lost your key and you are still using the OS that it is tied to you can use a tool like Magical Jelly Bean that can retrieve it for you.
Once you have an ISO downloaded you can either burn the image to a DVD, install from a USB device, or even use Microsoft’s own USB tool.