Undeletable folders in C:\, please help

I'm facing this problem:

I made a clean re-installation of Windows and everything has gone smoothly so far. I even renamed user accounts and also the user names, as well as the name of the PC that was given by Windows.

I have a directory (with sub-directories) in C:\ which mostly contains LibreOffice documents, a few Excel spreadsheets and PDF files. This directory and a few others have survived the Windows re-installation. The problem is that I cannot delete these directories even though I try to do so as administrator. The Security tab in the sub-directories properties shows for a few of them in the "Group or user names" section numbers that begin with S-1-15-21-1461528909.... (I cannot read the rest of the string). There are about 12 to 13 such entries for the sub-directories affected. Some entries begin with "Account Unknown" and then the string of numbers.

Windows 10 Pro v.2004.

You need to take ownership of the folders:
Change Owner of File, Folder, Drive, or Registry Key in Windows 10

Thank you for the suggestion. I think that I panicked too soon! Those strange usernames did it...

In the end, before seeing your suggestion I tried a solution from command prompt that is I first entered dir /x to see the directory and then
rmdir /Q /S followed by the problematic directory name.

I was surprised that it worked with no problem.

I'm not quite sure about what I did, but I started with this tutorial and then when I started the Windows installation the options to delete my apps and personal data were not exactly what I had expected. However, I continued and in the end all my programs were gone as also my documents. A few directories that I had created in C:\ remained intact.

The tutorial which you quoted has four options,one of which is Rufus.Can you remember the choice you made?

I used Option 1.