Ok, so assuming my C:/D: drive are partitioned on the same hard-drive (how can I check?), I can delete all of D, extend C without any chance of losing data, and voila? I can install mint 7? Maybe Mint 7 KDE. On either C or D?
Easiest way to check this IMO would be to create a Mint LiveCD and use gparted. If you have two drives, the gparted screen will show /dev/sd* in the top right corner (*denotes a letter, usually a), and the entire disk partitioned in one big partition, and you will have the option via drop-down menu to show /dev/sd? (? being a different letter than *, probably b).
If you have one drive that's been partitioned into two partitions, you will see only /dev/sd* in the dropdown menu (make sure you don't have any USB drives in, of course, because gparted will read those too, and might screw you up), and the drive will be split in half or 3/4 and 1/4, or any variant thereof.
If you have two drives, simply choose /dev/sd? from the dropdown menu (make sure there's no data on it first!) and delete the parition there. During the install, you can choose to install Mint KDE on /dev/sd?.
If you have one drive with two partitions, delete the second partition on /dev/sd*, and choose to install Mint KDE there during the install.
Do you know which version (either Mint 7, Mint 7 KDE, or perhaps Mint XFCE) you want?
"People should better think of their computing devices as facilities lended by the DHS." -Berke Durak, Linux wipe (secure file deletion) development.