Memory space vanishing

Hello geeks, and thank you for your help in advance. I apologize if this is the wrong section, but I honestly don't know what kind of problem this is.

For some reason, my computer's memory has been gradually reducing over the last couple of months, without me installing any major programs. Multiple gigabytes would just be reduced at once, as if the computer installed a major game, but with no apparent cause. I tried malware and rootkit removal, and those came up clean. I scanned memory space with Total Commander, and out of a half-terabyte memory, there's around 160 GB not accounted for by the C Drive and remaining memory. Where is all of this space, what is it being used for, and how can be recovered?