Right!, let's start at the end.
I'm surprised that you have drives A: and B: because they were usually used for floppy drives. Normally it's C: and D: with the CD/DVD being E:
There isn't two physical drives, It's one disk with two partitions, one is the one that the operating system will be on and the other is probably for other files.
If you go to 'Start', 'My Computer' and Right Click the mouse over one of the drives, then select 'Properties' you will get a display that shows how much space is used and how much is free. This will tell you how 'full' the drives are.
If you want to transfer your music and photos, you may be better off getting an external hard drive, which plugs into a USB port (and will probably become drive F:). You can then 'move' or 'copy' the files to that in 'My Computer'.
If you 'move' them they will be taken off your initial drive when you do that, if you 'copy' them they will be duplicated and then you can delete them from the original drive. (My personal preference is to copy them,then check they are there and finally delete the originals.
I hope this makes sense, good luck