The full error message you might see is: "Windows installation encountered an unexpected error. Verify that the installation sources are accessible, and restart the installation. Error code: 0xE0000100"
Solution: Check if the RAM from your server fits the minimum requirements. The minimum requirements according to the Microsoft documentation are 512 MB. However the error message will appear if you install Windows 2012 R2 as Virtual Machine with 512 MB. If you raise the RAM temp. to 1 GB you can install the virtual machine and later one limit the RAM.