I've found the solution; needless to say, not on MS Knowledge Base. A chap called Petter Nordahl-Hagen has produced a utility called "Offline NT Password & Registry Editor
It's a live Linux CD or USB stick, and it will modify the SAM
file to change or remove the password of a user or administrator, including the
administrator. There was a warning about changing the password not working on XP and indeed I found that this didn't work, so I removed the password on the administrator account, booted windows and got back in. It was then just a matter of changing the passwords for all accounts and it was job done, reinstallation not necessary.
The whole utility weighs in at 4.1 MB. I'm very impressed. One for every sys-admin who has to take a trip to the dark side now and again.