Well...assuming the hardware is all good there is the possibility that space on the C drive had been commandeered for nefarious purposes: storage for illegal downloads, bot attacks, that kind of thing.
You might revisit your wifi security. There have been instances where a desktop owner gets busted for porn downloads that were in fact executed by a neighbor who had access to the wifi.
And test all your ports at the "shields up!" utility at
https://www.grc.com/intro.htm .
There are other more exotic things that I can think of but they are speculation on my part. Win 10 tries at all costs to tie you into the internet and various on line accounts. It seems likely you're going to have to reinstall OS on C drive again, if you do I suggest a LOCAL setup where you unplug from the ethernet cable before proceeding with the install, and systematically ignore Microsoft's efforts to persuade you that you'll have a "better experience" if you connect to an account on the web. You're half way there when for security verification they offer three local checks (name of your dog, etc) rather than hooking into an account that sends a text message (to allow you to get on to your own desktop!).
Assuming that you have your personal data files stored on a different drive you'll want to run it through at least two different AV checks.
There may be innocent explanations if you're a gamer for instance you may have inadvertently created a massive download related to recording a gaming experience or some other thing. I was surprised to see that my son had something like 35 gigs of game related Overwatch stuff on the upstairs computer. Fortunately the bulk of it is on the HDD. In any case the OS up there was comfortably lodged on a 128 gig SSD but when I looked at it again it was 90% full and showing a bright red bar. I mirrored it on to a 500 gig SSD just last night and things are looking better.
The probability of some of these sorts of things has to be assessed against the C: drive size and the kind of uses to which you put the computer. If you were economy minded and had a 128 gig SSD for the C drive or even 250 its not hard to see that it could fill up over time. But I have known people whose desktops had been turned into servers for use by bad guys.
Greg N
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