While a true 500MB ISO is rare for a complete desktop OS, these "Super Lite" editions are the closest alternatives for low-resource hardware: The FASTEST Windows 10 ISO? - AtomOS 10 Lite

A fully functional, official Windows 10 installation cannot fit into a 500MB ISO, as standard versions typically range from 3.9 GB to 5.8 GB. However, you can find highly modified "Super Lite" community versions, such as , AtomOS , and Windows 10 Superlite X , which strip away non-essential components like Microsoft Edge and Cortana to achieve ISO sizes between 700MB and 2GB. Top Lightweight Windows 10 ISOs