Start Menu "Open", "Open All Users", etc. failing

> On one of my XP Pro SP2 machines, right-clicking a folder in the Start
> Menu
> and selecting any of the options "Open" "Open All Users" "Explore"
> "Explore
> All Users" fails to do anything at all. It should open/explore the
> corresponding folder, e.g., c:\documents and settings\all users\start
> menu\subfolder. It does so on several other similar machines, but not
> this
> one. On the problem machine, these selections work only with the Start
> Button itself, never any folder in the Start Menu. The problem occurs for
> all users and for a newly created user; and whether you use the classic
> start
> menu or the new start panel under "all programs." Selecting "rename" and
> "delete" and "sort by name" and "properties" work normally. As do all
> context-menu options for individual files (as opposed to folders). No
> errors
> appear in the Event Viewer, and all other functions of Windows appear to
> be
> normal. c:\windows\explorer.exe and explorer.scf appear to be identical
> to
> those files on a working machine.
>
> I've hunted through the Internet and hunted through the registry looking
> for
> how to fix this problem or any further information on how those menu
> options
> work -- anything that would help debug and analyse the problem. If you
> have
> any relevant information for me, I will be very grateful for it.