Well Brink, this is very weird. I've always used that Tute for this purpose. I hadn't noticed but in XP's registry my HKLM\System\MountedDevices where I had the Offline key had become MountedDevices(1) and there was a second MountedDevices below that without the (1).
I stuck my neck out and just deleted the (1) key altogether and reentered the Offline key in the second MountedDevices with DWord \DosDevices\D:, \DosDevices\M: and \DosDevices\N:
each with a value of 1, rebooted and now the darned things are well and truly hidden.
I have no idea how that happened.