If Ubuntu is installed on the logical 'D:\' volume; when you are running Ubuntu, you ARE accessing the D drive and it IS mounted. It's just not called 'D:\'
The thing to bear in mind here is that the various Linux file-systems are completely different to the NTFS/FAT based file-systems used by Windows. Also different drives/volumes in Linux are not assigned letters like in Windows. So when you are logged into Ubuntu, the root of your 'D:\' partition will be the '/' directory, this is called the root directory. All of the subdirectories off of root '/bin/', '/home/', etc are also technically all on the 'D:\' too. As I said, it's just not called 'D:/'
Linux treats pretty much everything as a file. Any devices attached to a Linux system will have a file-entry somewhere in the '/dev/' folder. To mount the 'C:\' manually, you'd have to identify which file in '/dev/' relates to your drive/volume. Typically drives/volumes are usually given names like sda1, or sdb1 in /dev/.
After identifying which of the entries relates to your 'C:\' partition, you could manually mount it. But as Ubuntu auto-mounts any attached devices/filesystems that are present, your 'C:\' should be auto-mounted anyway when you log in and should appear as an icon on the desktop.
So from Ubuntu you should easily be able to access your windows partition and manipulate the files on it.
But AFAIK, Windows cannot mount, read or write to anything other than NTFS or FAT based file-systems. And the partition you have Ubuntu on will be formatted in ext3 or ext4, which are Linux/Unix file-systems which cannot be read by Windows. This is why you are unable to read anything on the Ubuntu partition when logged into Windows.
Windows will be able to see the 'D:\' partition, but because it contains an incompatible file-system, Windows will not be able to access it.
So to sum up, you can access your Windows partition from Ubuntu, but you won't be able to access your Ubuntu partition from Windows!
So you could just keep all of the documents you want to share between Ubuntu and Windows on the Windows partition. However...I'm not sure if the situation has changed, but NTFS support in Linux has always been a little buggy and sometimes prone to error (it can read NTFS OK, but has problems writing to NTFS sometimes). So if you want to share documents and media between Linux and Windows without risking corrupting or losing data; you might want to consider setting up an additional data partition formatted in FAT32 to store documents you want to share, or use a USB stick, or some other removable media using FAT32 .
This way any documents you want to share between the two OS'es can be placed in the third partition (or on the USB device). And because it's FAT32, it can be mounted and used by Linux and Windows without any problems.
I hope I've explained things clearly enough!
