Let's see if I can help a bit:
Right now, it is likely that you have 1 physical hard drive in your computer that has been split into two drives - C and D. It's also likely that you can easily add a second physical hard dive to your computer. If so, you are in luck. All you have to do is buy a new hard drive, pop it in the computer, and bingo - you have tons of extra space. Your old C and D drives are still there, and now you have a new E drive to fill up.
If you are feeling adventurous, you can do it yourself. Folks on this board can give you some hand-holding and walk you through the process. Or, if you have a friend who's at all handy with computer, they can do it quick and easily. To be stereotypical for a second, most guys would love to help you out here.
But the easiest approach is to let the store do it for you. If you buy the hard drive at someplace like BestBuy, they will install it for a reasonable fee - they even have "free installation" specials from time to time.
Now, there is the chance you can't just pop in a new drive - if so you are going to have to move over your files. Again, the easiest approach is to just let the store do it for you. They have all the software - it's probably not worth buying it yourself.