This free utility provides a simple interface that lets you select the version of Windows you want, then downloads an ISO for that version straight from Microsoft's download servers. Enter the Microsoft Windows and Office ISO Download Tool. The ISOs can be hard to find, though, and for versions of Windows other than the most current, the site tries really hard to push you into using the Media Creation Tool instead. Instead, they're stored on its TechBench site.

Microsoft used to make all these ISOs available through a site called Digital River, but it doesn't anymore. Download Any Windows or Office ISO Using a Free Third-Party Tool