
In other words, all of the bloat seems to be Windows. With a tiny bit in ProgramData - you can drill down in that folder and it will quickly be apparent which application that is.
And in the windows, the winsxs seems to be the thing.
Based on the description, this is something that may grow every time Windows applies an automatic update. If it's a pig, this may explain the shrinking disk space.
Not sure how much more I can help. Hopefully the winsxs cleanup that Murrin pointed you to works. Other than that, I would suggest saving some snapshots of your WinDirStat. Later on, if the free space is shrinking, you can start it up again, and maybe notice which folders have grown appreciably.
This hasn't come up, but you can tune the amount of disk space your browser uses for caching temporary files. This limits the maximum amount of space it will use. In IE, it's Tools > Internet Options > Browsing History > Settings > Disk Space to use. This should be set to a couple of hundred MB. Probably is tho. But I figure you're wanting to learn stuff.