Step 1, Boot your computer using your Bootable Media (DVD or USB media)
Step 2, At the Install screen, press SHIFT+F10 to access the Command Window.
Step 3, Run diskpart command and type the following command:
list disk.
select disk 0 (where 0 is the disk number)
clean.
(CAUTION:This will remove all partitions on the selected disk causing data lost.)
convert gpt.
create partition efi size=512
create partition primary size=40960
format fs=NTFS quick.
assign letter=W
Then type exit to completed diskpart
Step 4, Type mkdir w:\vdisk
Step 5, Run diskpart command again with below commands
create vdisk file=”w:\vdisk\windows10.vhdx” maximum=25600 type=fixed
attach vdisk
exit
Step 6, exit the command window and click install now with "Custom: Install Windows only" and selct the vdisk just attached.
Step 7, Windows is now installing the operating system.