You have a single HDD which is partitioned. Acer seem to do this a lot and my guess is that they are equal partitions in size. It would have been better if Acer had set the C partition at say 20GB and left the remainder as Data D. That said the principle of keeping the operating system on the C partition and all your data, (Music, Pictures & Documents), on D is sound. In the event of a system failure you can restore the OS to C drive and your data should be intact on the other partition.
This assumes that the HDD doesn`t fail of course. The answer I think you need is to keep data on the D partitionso it is all in one place. I would relocate "My Documents" "My Music" etc to this drive.
I would also use a Partition Manager to reduce the size of the C drive to 20GB and free up more space for the data.