You can see the Windows files, because you can see that partition; they wouldn't be copied across. You can't see Ubuntu when you're in Windows, but you can see Windows from Ubuntu. Mounting just means removing access to that partition. It's a very minor operation that writes a tiny amount of data into the directory called /media. In Ubuntu, the path to the mount points is /media, so for Windows it's /media/OS. You need to look at the table to see what's going on: $ sudo fdisk -l
sudo mount -t ntfs /dev/sda2 /media/OS
Notes: the command is umount, not unmount. The file sys type for Windows is network file sys. The OS is a directory in media, it could be called anything you want, but OS would be the default. sda2 might not be correct for you; have to look at you partition table.
sudo umount /media/OS
Actually, I don't see any reason why you would bother umount/ing the Windows partition, but you can if you want.