If C is your system drive and you format it, you will have an expensive brick. The system will not run.
I'm going to make a few assumptions / ask questions in interpreting this, please let me know if I am right. And remember, Google Translate is your friend.
Your system is installed on your C drive.
You have less free space on the C drive than you should.
You have accounted for the total space taken up on the C drive and the free space does not make sense.
How big is your C drive?
How much space does it show as being free?
How much space do you think that you have used?
How much space do you think is missing?
Are you aware of storage space used by the page file, hibernate file, and system restore points?
And, finally: Can you download WinDirStat, run it, turn on the option to display unknown space, and either find the problem yourself or post a screen shot?