System administrators and university servers often use these open directories to host large files like operating system images (ISOs). Finding an "Index of Windows 7 ISO" essentially means you've found a direct file repository where the installation media is stored. Why Do People Search for These Indices?
Instead of scouring obscure server indices, consider these more reliable methods:
This is a safer "public directory." Many users have uploaded archived versions of original Windows 7 retail discs. Because it is a library, you can often see user reviews and verify the authenticity of the upload.