This effectively stops the Task Manager from running... there were about 30-40 similar entries which unfortunately I didn't make a note of (I was repeatedly hitting delete, enter, up, (pause), delete, enter, up... in regedit), but containing common anti-virus/management tools like procexp.exe, nod32 etc.
The only key which should normally exist and might contain the Debugger -> ntsd -d value should be "Your Image File Name Here without a path", I think.
It'd be nice if Spybot would search for such patterns and offer to delete the keys, since it took me quite some time to stumble upon the cause and solution