Hi and welcome to the forum
The best thing to do is move all data on D: disk, leaving the C: disk for operative system and programs only
So, you can move all your data like pictures, music, etc on the D: disk.
However, it seems that you have rather old drives. Maybe is a good idea to get a newer drive. Nowadays you can find a 640GB drive at a reasonable price
There are software like Acronis True Image that are able to transfer all the contents of a hard drive onto a new drive, so you can transfer all the contents of the C: disk on the new drive without being forced to reinstall again everything