A hard drive can be setup any number of ways. It really depends on how the user wants to work. Some people setup a single large partition and put everything on it. Others setup one of more additional partitions. This can be done to seperate data from the operating system and programs, to create a temporary download or work space, any any other number of reasons. I've seen systems that have multiple partions so that:
C: = Operating System
D: = Programs (I don't agree with seperating programs from the operating system)
E: = Downloads
F: = Personal Data
G: = Temp space for burning DVDs and CDs (helps keep the space unfragmented)
H: = Work space for video editing
(Drive letters may not be exactly what I've seen; example only).
People do this so that if they have a problem with corruption on the drive or the operating system, they can just reformat that one drive and not lose everything on the other drives (which would happen if everything were stored on the same large partition). Sometimes multiple physical hard disks are used so C & D are on one, E & F on another, etc.
What works for you depends on how you like to work and what you work with. 15GB for C may be just find. Or it may be far more than you need or way too little.
