In the days when Windows was easier to crack, it was not uncommon to find very small, specialised ISOs of "miniature windows" designed specifically for running a certain task -- typically some form of data-recovery app, system repair/maintenance or similar -- and they would fit on a single CD-R, have almost no system requirements, and, if I recall successfully, be bloody useful to my school's computer administrators who otherwise just had to reimage machines all the time with Norton Ghost.
Nowadays it seems that a Windows ISO does not come up short of 4 GB and this tradition of "repacking" them into small, optimised, crap-free operating systems (albeit of questionable legality, but inevitably used on a machine that had a real windows license!) has been lost. I lament this.