Read these two links.
For most, that's it.
I just bought a Lenovo Y550 with a 400 gb hard drive. It started with around 390 gb which was fine because I know that that missing space is the operating system. But as I was installing programs and downloading stuff I began to randomly lose several gigabytes every now and then on my hard drive. I am down to 330 gb and I can only account for at most 20 gb meaning I'm missing nearly 40 gb. Could someone give me a clue as to what is happening and how to get the space back?