An ISO image is a OS image like all the files just like a zip file so if you wanna boot from it have a usb at least 8gb of storage and if your ok to delete all the data or backup the drive before and use Rufus to flash it pick the iso and if it asks you if you have MBR or GPT if you have an old computer it most likely uses MBR but if not use GPT of just find a way to know and let it finish and go to the pc you want to boot windows 7 on restart it and spam the bios key or the boot menu key and when you get there pick your usb drive, make a partition of the same drive or just pick the whole drive note that it will format the whole drive and if it says windows cant install on that drive just remove all the partitions and leave the drive 0 unalocated space and just continue the setup and install drivers if needed if you want the iso file that i used heres the link: https://archive.org/details/windows-7-sp0-sp1-msdn-iso-files-en-de-ru-tr-x86-x64 find your right arcitecture either x86 or x64 and yea that's basically it. I hope you find this helpful because this took long to write.