Drive containing Windows getting filled automatically without any downloading.

I have installed Windows in my C Drive as most of the people do. But the problem is, I don't download any games, music or videos or even text files but my C drive is getting filled automatically. When I installed the Windows, it was having 25GB free space but now only 3GB free space is remaining. I usually delete Temporary Files but still the system is getting slow and no space is left in C drive. Please help what to do now.
