You need to find what is taking up so much space..
You can start in the Add/Remove Programs in control panel
To find the rest you'll just have to right-click folders and go to properties and see how much space its using, and keep searching until you find exactly what it is using up so much space. Windows does not use 15GB, you have some big files/programs somewhere.
Keep in mind that simply removing unused files won't speed up windows tremendously. What you need to do is free up CPU usage, RAM usage, and optimize your windows settings for performance. If you right-click on the taskbar (The bar along the bottom of the screen by default) and go to Task Manager, you can click on the 'Processes' tab and see how much stuff is actually running and in-use. Windows only NEEDS 12 processes.. so that gives you an idea of how much extra stuff you have running on that computer. Under 40 processes is ok, under 30 is ideal. The number doesn't really mean anything though, its all about cpu usage and memory usage and how far you're willing to go to make xp fast.
If you're not very computer savvy this doesn't sound like a job for you. Perhaps you know a good friend that works on computers for a living and he can help you out a bit.
If you're trying it on your own and learning as your doing, you will more than likely make mistakes and make the operating system unusable, or simply remove something you didn't want to and then regret it.