Cannot take ownership of any folder, despite repeated attempts

Sometime last week -- and I can't figure out when or why -- I lost ownership of all of the folders on my PC. I noticed when I was no longer able to create folders. Then I discovered none of the folders have security tabs.

The C drive itself and my external hard dive (L) both have security tabs, but when I go into them to take ownership (Properties -> Security -> Advanced -> Check both "Replace owner..." and "Replace child permissions..." -> Apply) the process fails because of a "Failed to enumerate objects in the container. Access is denied" error.

My system restore didn't fix it, because it didn't go back far enough (I think), and I can't find the same error in the various forums.

Does anyone have a suggestion beyond reinstalling/refreshing Windows? I could do that, but reinstalling all of my software is five or six hours I'd prefer to waste on just about anything else.

Thanks in advance!

Strangely, that didn't work at all. I watched the notifications go by, but none of the folder permissions changed. The notifications said repeatedly "Access is denied."