Just in case you need to reinstall from a clean and up to date disk, or your recovery partition has been killed - since we've just learned that Dell doesn't provide replacement disks - you'll find Windows 7 iso disk images here:
http://www.heidoc.net/joomla/technology-science/microsoft/14-windows-7-direct-download-links
The images are hosted by Digital River, who also run the Microsoft Store. So there's no legal issue here, and you can also be sure that the disks haven't been tampered with.