How can I quit Nautilus? Supposedly it's Xubuntu ( with XFCE ); there are no panels, but right-clicking provides an Applications-Help-Reset-Exit-etc menu. After some googling Nautilus taking over the desktop seems to be a common problem, and typing something like killall nautilus or nautilus -q in Terminal is recommended. How can I open Terminal with no right-click menu and no panels, and Alt+F2, which, as far as I've understood, opens the Terminal, doesn't work? Any other shortcuts?
Thanks in advance and sorry for a (probably) silly question =)

Try switching to tty1 console by issuing Ctrl-Alt-F1, logging in as your user or root, and do the "kill" procedures in there. then, to switch back to your desktop, press Alt-F7.
don't use nautilus in xfce, it will always act like that.

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Did you remove your panel or loose it? There should be a panel in Xfce. Check the configuration or settings entry in the menu for getting it back in the latter case.

The default file manager for Xfce is Thunar, which is not very different from Nautilus and won't give trouble.

Because nautilus is the desktop manager for gnome, running nautilus in xfce will create another desktop on top of xfce's desktop, to prevent this run it with the option: