Open Windows Explorer (Windows Key + E Key) and you should see the C & D drives in the left hand pane. First check how big the drives are by right clicking on each and select properties - this will tell you how much space you have free on each.
I suspect you have quite a lot of music files on the C drive so I would move as much as you can to the D drive. Create a folder on the D drive called Music and then drag and drop folders of music into it.
You need to have about 15% of your C drive available as free space for XP to run properly and the first thing I would do after transferring files would be to run Ccleaner click here Next to a defragment of the C drive in safe mode. Then start a ruthless deletion of surplus to requirement files, unused programs and downloaded installation files.
You will have to get Windows Media Player to monitor your new music folder otherwise the tracks won`t be playable.