QueenVixie It's not a command, it's basically reinstalling Windows, except not all of it and without erasing your drive. I'm sorry to keep coming back to this point, but please please create an ISO, as in option two, run the setup.exe, and let it repair Windows that way.
Part of the reason for this is that the other option here is to manually replace the file in question. But a) as I said, changing permissions on a Windows folder is dicey; and b), without another Windows PC to donate a copy of the file, you're stuck downloading one from the internet, which is even more dicey. I'm not saying you can't do it, I'm saying I wouldn't recommend it unless you were out of options.
It's your computer though, and your choice.
https://www.winhelponline.com/blog/securityhealthsystray-bad-image-0xc000012f/