This is from Microsoft's website. (their github)
FAQ Setting up Raspberry Pi
I can't download the Windows 10 IoT Core Insider Preview image for Raspberry Pi. I see a blank screen.
We release the Windows 10 IoT Core for Raspberry Pi 2 through our program on Microsoft Connect, which also provides a platform for bug reporting and providing feedback. To be able to download the image, you need to be signed up for our program on Microsoft Connect and signed in. If you have not signed up for our program on Microsoft Connect yet, please click here to sign up. You can find step-by-step instructions for signing up or using Microsoft Connect here. If you have already signed up for this program on Microsoft Connect, and try to sign up again, don't worry. You will just see a blank page.
Please make sure you are signed in to Microsoft Connect before trying to download the image.
Why does my screen appear cropped or stretched?
This can happen on some monitors. To work around this force the board into DVI mode (vs. HDMI mode). You can read more about this here. Mount the SD card to a local PC and proceed to edit the file in the EFIESP partition. For example if the partition is mark as H drive, edit H:\config.txt by adding the following line.
hdmi_group=2 # forces DVI timing to be used
Can I set up a Raspberry Pi 2 using Windows 8.1?
You will need Windows 10 on your PC to be able to set up Raspberry Pi 2 running Windows 10 IoT Core.
Why does my SD card have to be 8gb, do you really need all that space?
The IoT Core image is under 1gb, the 8gb is meant to give you space to install additional content on your device.