I think I need to familerise myself with partioning, I'm in the dark here.
I won't try to explain partitioning here, but I can make a few relevant points to your situation.
The problem you are seeing is that there is no free space available in which to install Mint. This is normal, as windows will use all free space.
There are a number of ways to create this free space and the installer can't know which one is safe for your situation or suits you, this is why the installer offers a number of options.
Windows refers to partitions as ;C,;D, etc, this is useless info, and why we need reliable info (from gparted, fdisk or parted for example).
Once the free space is created you have the option to manually setup partitions as you like, or allow the Linux installer to do it for you (most Linux installers do a good job of this).