Why does the storage space consumption of my SSD fluctuate?

Can someone help me with the following problem?

I noticed today that the free space on my hard drive fluctuates. These are fluctuations to an alarming extent. I don't mean that kb's are moved, but Mb's and even Gb's. The computer ran for 3 days in a row until yesterday and yesterday evening I got the message that the hard drive was full. And suddenly there was actually over 230 GB on partition C (Windows and programs only) on the other partition nothing happens. After restarting the PC everything was back to normal. The memory still available is currently 75.8 GB. This morning it was 76.0 GB.

I also noticed that my Wi-Fi reception was terribly poor. The connection was constantly breaking and even if it was stable, it was with hardly any juice on the line, while everything was fine with the laptop next to it.

At first glance these problems seem to have nothing to do with each other, but after my wife received mail from her bank with a new credit card because someone was supposedly trying to misuse her old one, I became a little suspicious. After a virus scan with 3 cleanings, the Wi-Fi signal is now stable again, but the storage space of the SSD continues to be fully loaded. However, this happens very slowly, so that analysis tools can hardly tell where this increase is coming from.

Has anyone experienced anything like this before? Can there be simple technical reasons for this (e.g. Some automatic Steam or Windows update?)

I'm slowly getting the feeling that someone from outside is trying to play their jokes with me… (virus or hacker?) And I have to say, if it was a virus, I removed it, but why does the SSD still fluctuate? Could it be that some hacker is using my PC as a "bot" to redirect data or something?

Does anyone have any tips or advice on how best to proceed?

Sounds like a virus, I would format the hard drive

No?

Temporary files would also recommend you to do as much as get to tmp files on your hdd otherwise your ssd will be rubbed through very quickly

If you actually have malware on your computer, 'cleaning' doesn't help either. In the best case scenario, a virus program can tell you that it would find something, 'cleaning' an infected system is an illusion. Once a system has been corrupted, it is no longer trustworthy and has to be set up again.

Fluctuations, even by a few GB, are completely normal, e.g. Due to the swap file. A difference of 70 or 80GB, however, is not.

How exactly do I do that?

Look up in programs which make such files or sometimes delete the tmp folder

Do you think simply resetting the system to factory settings would also be enough? I can never find the Windows install disk and I want to keep win10pro ^^

Hm has unfortunately not brought anything memory continues to drop anyway thanks, I will probably not get around formatting

Unlikely, just make an installation stick using Microsoft's Media Creation Tool