This is the first time I have blogged here about something other than running with least privilege. It's about a neat trick, though, that can be useful for some people.
If you need to shut down the main Explorer process, you could just kill it from Task Manager or Process Explorer. But undesirable and unpredictable things can happen when you abruptly kill any process, particularly one as central as Explorer.
In Windows XP, you can get Explorer to exit cleanly by getting to the shutdown dialog (e.g., Start / Turn Off Computer, or Start/Shutdown), then hold down the Ctrl+Alt+Shift keys and click the "Cancel" button. (Ref: JeffDav's blog.)
In Windows Vista with its standard Start Menu, click on the Start button. Hold down Ctrl+Shift and right-click on any empty area of the menu or on the power/lock buttons at the bottom of the right half of the menu. One of the context menu choices is "Exit Explorer". Choose this and the main Explorer process will cleanly shut itself down. (Thanks to Mike Sheldon and Raymond Chen for this tip.)
If you are using the "Classic Start Menu" option in Vista, the XP Ctrl+Alt+Shift+Cancel method still works.
OK, so chances are right now you're looking at nothing but wallpaper and the Sidebar and wondering, "What do I do now?" There's no Start menu anymore, and Win+R doesn't display the Run dialog. Answer: press Ctrl+Shift+Esc. This starts Task Manager. In Task Manager, choose "File / New Task (Run)", type "Explorer" and click OK. The shell will come back to life.
Note that on both Windows XP and Windows Vista, only the "main" Explorer process exits – that is, the process that manages the Start menu, taskbar, and desktop. With default settings, all Explorer folder windows are managed by that process as well, and so they will close too. However, if you have configured Explorer to "launch folder windows in a separate process", then those folder windows will not close when you apply this trick. Furthermore, when I tried this on Windows XP, I needed to manually close all those folder windows before running a new instance of Explorer would display the taskbar, etc., instead of just displaying yet another folder window.
Why is this hidden nugget even there? Its purpose is to help developers and testers who work on shell extensions to be able to stop and restart Explorer quickly and cleanly without having to log out.
Obviously, though, this trick can also be used to launch Explorer elevated. If you've exited the shell process and start Explorer from an elevated context, the entire desktop shell will run elevated. I cannot say this without adding caveats. If you do this, everything you start from this point on will run elevated. Shell extensions will run elevated, including the ones with serious security flaws. If you shut down Explorer again, any child processes that were launched will continue to run elevated, including browsers, IM clients, etc., with all the risk that incurs. IE Protected Mode does not operate when IE is running elevated. Less important but also significant is that any processes running at Medium IL will not be able to interact with the elevated shell – for example, to display taskbar notification icons. In general, because Explorer was neither designed for nor tested with this kind of elevated execution, you should not assume that anything will work correctly, including something as fundamental as user logoff. If you really need an elevated Explorer window on Vista, you can try the unsupported trick I described in this post instead of elevating the entire shell.