|Are you sure that D isn't your recovery partition? What is the capacity of D?|
When using an OEM computer like your laptop it is difficult to change the way partitions are set up. That said, it is very easy to store your personal files anywhere you want to.
Assuming the D drive is a storage partition and NOT the restore partition you can simply change the default path where you save things. Usually, if you change a download path once it will use that path from then on.
Just create some folders and give them descriptive names.
For personal files that are already stored on the C partition you can move those files too. If they are important files I would recommend that you use the COPY and then PASTE options instead of CUT and PASTE. After copying and verifying the files in the new location work then you can delete the files on the C drive.
If you only have ONE copy of your personal files you are gambling. Things happen. You should keep at least two copies of data that you can't replace.
Options are external hard drives or my preference, Optical disk, like DVDR or CDR. These disks are more permanent and cheap to make multiple copies.