I can't terminate some processes. It says "Access denied". I am the ultimate owner of the computer. How can I terminate such processed anyway?
These are not system processes, these are file managers during file deletion.
Sometimes task manager doesn't kill the process. It's also possible to kill processes from command line with 'taskkill'.
It might require to start 'cmd.exe' as Administrator. This can be done at least in Windows 7 by clicking Windows-logo, typing 'cmd.exe', right click the icon that appears, select 'Run as Administrator'.
I've seen also processes that this procedure wasn't able to kill. Microsoft documentation of taskkill