sounds like one of the frontends set itself as your system shell. From safemode your going to have to edit your registry to point back to explorer.exe.
Perhaps someone else on the site has a quick registry entry you could just import that would do this automatically? Sorry I dont have anything more to help but that.
I remember this happening to me a LONG time ago when i was booting into a desktop frontend (like LCARS or something) and wanted to switch back to regular windows explorer and they directions showed me how to switch the command to open explorer on boot.
Hopefully someone else has some more info to elaborate what im trying to say.