C:\Windows is 33.4GB, C:\Program Files is 3.18GB, C:\Program Files (x86) is 5.58GB. Yes, just one partition. Yes, I have run the built in cleanup; yes, I downloaded and ran CCleaner. I also disabled hibernation just now and went from 12GB free to 17GB free. I will download TreeSize Free and analyze further. Thank you so much!
If you now have only 17 free on a 500 GB drive, I would certainly try to gain more space.
How much of the space is your "personal data", broadly speaking? Pictures, videos, Word files, Excel files, whatever. It's typically video files that really eat up space. This stuff is usually somewhere under C:\users in a standard installation.
You may have an unnecessarily high amount of space devoted to System Restore, but that can be reduced to say 6 or 8 GB if needed.
Likewise, your page file may be larger than required. You could cut it down to 1 GB.
But if you have say 350 GB or more of personal data, I'd probably start to think about getting a larger hard drive.
My Windows, Program Files, and Program Files (x86) folders total about 30 GB compared to your about 40 GB. I run a fairly lean system, so your numbers aren't outlandish.
The "appdata" file can get large in some circumstances, but I wouldn't get excited about that yet, pending knowing how much personal data you have.