I have been using Ubuntu since a long time. I used to dual-boot it with Windows XP, which was very easy to set up via Wubi.
Now, with Windows 8.1, I'm unable to install on DOSS partitions. How can I set up the dual-boot?
Using "gparted" (Live ubuntu (sudo apt-get install gparted)) (or any disk partitioning tool), clear out one of the NTFS partitions to 'not partitioned', (hopefully that space is at least 18GB), and then proceed to the installation, Linux is usually installed on EXT4 partition and it is not for NTFS partitions. (Remember setting about 1GB of swapspace.) As long as GRUB detects windows, they won't work in each other's way.
From the "wubi" tag: "A way to dual boot Ubuntu and Windows without partitioning. Wubi will be discontinued as of Ubuntu 13.04 (Raring Ringtail). Please, consider using a regular install."
Back up data before partitioning that disk.
Get a version of Ubuntu that supports wubi (13.04) then go and install.