Drat - after doing a full backup with Macrium Reflect the free space on C: is back down to 124GB. What the bleep is going on here
I once used an A?V program called Avira (one of the best, free version, until they jacked with it).
After some 'upgrade/update', it began stashing large log files, not 20G/day, but too big. Over a few weeks to months these files would begin to choke the free drive space.
I tried relocating those files to another, much larger, drive (some of my machines have up to 8+ drives, RAID), and finally gave up using Avira.
The best A/V, firewall, plus, program suite is the free
Comodo Internet Suite (CIS); free version too.
Windows can create some update install/backup files that grow unchecked too, but Win 7-10 aren't as bad as before.
Your box is down around that 15% free space area where things will start to get 'funny'. Lots of little glitches and holdups that are never quite apparent, things just slow down or whatever.
One thought is your pagefile management, is it set to Window's 'recommended-auto' size? Do you have enough RAM? (I usually max out the slots, mem is cheap and does make a performance diff).
If you have
another drive that can be installed into the box/system,
you can move certain things, like the uber-used pagefile, to it; and get them off of the OS system drive. There will be a very noticeable improvement in performance.
A changeover to an SSD or hd-ssd-hybrid system drive would scream (but they still cost too much).