In April 2014, MS has updated Windows 8.1 to Windows 8.1 Update 1. (buildlab 6.3.9600.17031 and up).
One feature which was not very much marketed was the Windows Image File Boot also named Wimboot.
You can read more here on MS Web site.
In short, here is what MS says :
Windows image file boot (WIMBoot) lets you set up a PC so that it runs directly from a compressed Windows image file (WIM file).
WIMBoot can significantly reduce the amount of space used by Windows files. This helps you provide more free space to users, and helps you manufacture Windows on PCs with smaller drives.
So lets see how we can use this new feature :
We’ll assume you have already installed your Windows 8.1 (You can download an evaluation version here).
1-Lets make a WINPE out of the Windows 8.1 update 1 iso/dvd (I use QuickPE).
2-Boot onto this Winpe (I use rufus to « burn » the winpe iso onto USB).
3-Capture the installation : dism /capture-image /imagefile:e:\install.Wim /capturedir:c:\ /name:install /wimboot
Note here : e:\ is a second partition on my hard drive
4-Format C drive (so that we start from a fresh drive)
5-Apply our wim file (from step 3) to my C drive : dism /apply-image /imagefile:e:\install.Wim /index:1 /applydir:c:\ /wimboot
Note : after step 5, I had to do a bcdboot X:\windows /s X:\ where X is the partition where you just applied your WIM file.
This applies only if you have one unique boot & system partition which is nowadays rarely the case as Windows always created a hidden/reserved partition for the boot files.
And voila, you should end up with a C drive occupied by only 3GB (when it was about 20GB minimum before step 1).
This should work on X86 or X64 (tested), with a UEFI or MBR (tested) partition, with SSD drive or standard drive (not tested).
Here below a picture showing my final / wimboot setup.