OK, I am at a loss!
Can you do a (safe) experiment?
If you have an image or movie or whatever file that is relatively large, copy it and rename it and store it on C. My objective is to see if there is really free space on C, or if there is not and somwhow the windows explorer does not report it correctly.
So, copy a large file to C. Then, copy it again and rename it. Then, copy it again and rename it. This way, you can occupy space using a single file by renaming it. For example if you have 500MB file, you should be able to fit 28 of them on C before it fills up. So, you should easily be able to fit 2 or 3.
Can you give that a try?