I don't have any windows in my PC, actually I have uninstall windows 7 the other day.
Now I tried to install windows 10 and when it was ask to choose disk, by mistake I was just deleted the system disk (c) and now it is showing as unallocated space.
Therefore I can't choose next for installation of windows 10. and i'm unable to create new partition from 'Unallocated Space'
Shift + F10
would start command prompt and then you can run diskpart