This may only apply to XP SP1 (I haven't been able to test with anything else) but the DesktopProcess value isn't necessary. During some troubleshooting I fired up a copy of Regmon and traced the activity of the explorer.exe process... it doesn't even read it, and without it being present I'm still able to use explorer as a file manager under LS without the taskbar, etc. showing up.

The value may have been disabled in the SP due to the problems that it caused on certain systems. IIRC these involved the "explorer window opening on logon" problem and some errors with mapped drives.