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
When in task manager, on the bottom left is says: (the shield thing)show processes from all users. Click on that and after, more tasks will appear and then you will be able to delete the task you want!