In Folder Options, turn on: Show Hidden Files and Protected Operating System Files.
Download: TreeSize Portable Free & Run. Click on C: Drive to analyze. It will list all the large files/folders first.
Check the following files/folders, those can take a lot of space from your C drive:
C:\System Volume Information
C:\$Recycle.bin
C:\Pagefile.sys
C:\Hiberfil.sys if you have Hibernation turned on.
Windows is reporting that you have one or two pictures, no music, no videos, and a very small amount of documents.
The downloads folder is your largest media storage at this point and it's not even a gig.
Are you a spy sir? Have you encrypted all your material?
Four things come to mind:
A PC that has a lot of faults so lots and lots of space is being taken up by crash dumps. I sort of doubt this one or you would have complained about more than just not having space.
You have a virus, some sort of cryptolocker that is only working partially well. So it encrypted most of your files, but for whatever reason can't take the next step which is to send you the ransom, and can't otherwise take over your computer. I cannot recall the last time I saw a computer with this little media.
Someone is hiding something on your computer.
Your drive is failing.
It is very hard to fill up 500 GB without media. Even AutoCad files should get reported as media no?