Nano98: Windows 98 that boots and runs under 5MB (2003)
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.