dual boot - Windows partition recognized as unallocated space

I am new to Linux.
I was trying to dual boot windows 8.1 and ubuntu when I came across this: ubuntu installer won't recognize windows as an OS and the C drive is recognized as "unallocated space" by ubuntu. How can I install Ubuntu alongside windows? I tried installing before, when windows 8 was UEFI, I now reinstalled windows 8 and it's not UEFI now, what should I do?