C drive space disappearing

I'm constantly losing space on my C drive and the situation seems to be getting even worse recently: every time I use the computer I lose at least 0.5 GB of space on C, which means that getting on my computer twice a day costs me 1 GB of space. I keep deleting files or moving them to D. Usually, after an unpredictable amount of time, some space (but always less than what's been lost) is freed up for no reason that I can see: last week I had 6 GB free and then suddenly the free space was 10 GB even though I'd done nothing. But today the C drive was down to 2 GB even though I've moved files to D drive, and I'm getting desperate since moving stuff does nothing about the actual problem. I deleted 12 GB of photos, but all that free space has vanished.

I've tried some of the suggestions I've seen online, but the most space I've ever freed using cleanmgr has been less than the 0.5 GB I now lose every time I use the computer. I always clear my internet history when I close a browser. I just deleted 4 GB of old Windows Updates, but I've lost more space than that this week alone. The virus scan shows nothing. I'm wary of trying stuff like deleting System Restore; I'm a total amateur and my computer language isn't English, which makes instructions harder to follow since I have to try to interpret and translate terms I don't know.

What could explain the speed at which I'm losing so much C drive space? Why does some, but not all of it, reappear without me doing anything (this is something I haven't seen people mention when they write about disk space problems)?

I right-clicked Command Prompt to run it as an administrator, but when I wrote vssadmin list shadowstorage it replied that there had been an unexpected error. That might mean I have to use the Finnish language version of the command, but I haven't yet been able to find a translation by internet searches.

I installed WinDirStat. According to it, I have 301 GB in Users, 20.5 GB in Windows, 11 GB in Files and 14.1 GB in ProgramData/Program Files (x86)/Program Files, so the C drive is 346.6 GB. My D drive is 80.6 GB so C+D is 427.2 GB: though I can't remember the exact number, when I bought the computer it was said that it has around 500 GB space. 70 GB is missing, perhaps that is shadowstorage.

If you go to control panel/system/system protection/configure you should see a slider under "disk space usage".

How much is "current usage" next to that slider?

Thanks to the advice, I found System Protection for C. Current usage: 12,11 GB. Max usage: 3% (10.00 GB). Does this mean that shadowstorage only takes up 12 GB right now, or that there might be several restore points but the biggest and newest is 12 GB?

"Delete all restore points (this includes system settings and previous versions of files)": if it deletes everything including system settings and you can't select what happens, is it potentially risky?

I installed Treesize. For most folders it gives the same size as WinDirStat, but it also shows a Recovery folder with 164 MB (last changed 2010) and System Volume Information with 6.8 GB (last changed today 17.11.2014). According to Treesize's numbers, C+D drives is 439.5 GB.