That's the thing. Even though you think that you have completely shut down your Windows system, you really didn't. Windows keeps the drive mounted so it can speed up its boot process (this has nothing to do with fastboot, it's a normal Windows behavior since Windows 8).
Try turning off your Windows system by typing this into your
shutdown -s -t 0
This command will shut down your Windows system completely and you will be able to use the partition from the elementary installation.
Note that the command will shut down your system immediately. To set a time interval, you can change the
0 at the end to
30, which will shut down your system in 30 seconds. If you don't include the
-t 0 at the end of the command, I think that the default behavior is to set the time to 60 seconds.
As an alternative, you can mount the partition as read-only by typing this in your terminal:
sudo mkdir /media/Windows
sudo mount -o ro /dev/sdaX /media/Windows
You only have to execute the first command once.