: The "u" usually signifies a "Media Refresh" or "Universal" version, which often includes a small update (KB2533552) to prevent installation issues on some hardware. 24535 : A specific build or version identifier.
The long string of characters isn't just gibberish; it tells the story of exactly what you’re installing: win7-ult-sp1-x64-u-24535-esd.iso
Use 7-Zip or OSFMount to browse the ISO without booting. A legitimate Windows 7 ISO should contain: : The "u" usually signifies a "Media Refresh"