Why does my Windows hard disk appear to be full even if I don't have that much programs installed?

[Windows Central Question]

Why my windows hard disk appears to be full even I don't have that much programs installed?

My windows partition's free space changing without my control. How is that possible? Now I'm suffering from low disk space. But I did not install or copy to this partition anything recently. What's happening? How can I get rid of this problem?