Free application that will fix PC memory loss

When I reformatted my laptop, I made a separate memory; drive (c:) and drive (d:). In Drive (c:), I saved the OS and applications and in Drive (d:), I saved the documents, downloads, movies, photos, etc,. The drive (c:) used to have 55.1 GB of free space but now its only 53.2 GB, though I didn't save any files to drive (c:).
I just notice it whenever I go online. Is there anyone who knows what application I can use? Because every time I surf on the web, it eats up the memory in my hard disk? Where can I download it? And what's the name of the application? Thank you.
