Microsoft’s Windows 8.1 WIMBoot Lets You Install The OS In Only 3GB Of Disk Space
Bear in mind that this isn't a new version of Windows, it's just a new way of installing the OS. Instead of extracting all the individual Windows files from an image (WIM) file, WIMBoot allows them to remain compressed. From a user standpoint, everything still looks the same -- you'll see a C: volume with Windows, your apps, and all your data.
"So how does this work? Effectively, you copy the WIM file into a separate 'images' partition (just like you would for a recovery image), then use DISM to create pointer files from the standard C: operating system volume into the WIM file. These pointer files are completely transparent, and Windows knows how to boot the operating system (keeping all the files in the WIM) when configured in this setup," Microsoft explains.
If you want to try it yourself, you'll find detailed instructions in the ADK documentation.