DailyTech writes Microsoft has developed a 174MB build of Windows 8.1 that can run on Intel's Quark SoC. Designed for the Internet of Things, this operating system's goal is to get into common household and workplace objects, including appliances, clocks, cleaning equipment, printers, children's toys, shop tools, etc.
Currently, the operating system is only compatible with the Quark-based Intel Galileo, a $50 hobbyist board with a credit card sized form factor. The trimmed down version of Windows 8.1 is roughly 1/20th the size of an average Windows 8.1 install but it seems there's still a lot of work to be done as booting takes a whopping 2 minutes.
Unfortunately, this "non-commercial version of Windows 8.1" linked from Microsoft's Github bucket is not publicly available. It is currently only available to those that are approved to enroll in the IoT dev program. However, someone leaked the build in ZIP format to Kim Dotcom's encrypted filesharing service Mega (see this Beta Archive thread for more details).
It appears the build (Windows 9600.16384.x86 Windows Blue RTM IoT build 140731-1000 Galileo v1) weighs in at 174 MB. A typical Windows 8.1 installation consumes 3 GB of space, so this is nearly 1/20th the size of an average Windows 8.1 install. Boot time takes roughly 2 minutes, versus boot times of anywhere from 3 seconds to 30 seconds with Windows 8.1 on traditional computers.
The new Windows OS is designed to run on the Quark-powered Intel Galileo Gen. 1 hobbyist board. Amazon.com, Inc. (AMZN) currently sells the board for $50 USD (with free shipping) while Fry's sells it for $60 USD (the Gen. 1 model is no longer available on Newegg.com).