At work we had a problem with a pc where somehow the local administrator account was not accessible anymore, we didn't know the right password or something else was up.
Luckily I found this application: Offline NT password and registry editor
, which is a very cool linux bootdisk which can edit your registry, either to reset passwords, or to make other changes.
When changing a user, it offers good documentation and nice menus, which in our case found that the admin account had been locked, and it offered to unlock it and to reset all counters for it, and some other nice automated tricks.
Be aware of the catch that encrypted files for that user are not readable after this trick, but other then that it's a very useful tool.