I created the same problem by accidentally changing my administrator account to user account. I could not do anything. I tried the command prompt as administrator to activate the hidden Admin account as discussed. I called Dell paid support and spent an hour with them. He tried to do a system restore (via the menus, via Safe Boot, etc.), but I always got this error.
System Restore ERROR:: 0x80040154. I specifically asked him if I could Restore or repair from the Vista CD. He said no.
My computer has an administrator aco**** called RA Media Services, although it does not show up as a choice at login, only on the user accounts page. I asked Dell if there was a way to use this is as a "back door". I specifically asked two different tech support people if this account had anything to do with Remote Access. They said no, it must be an account left over from previous owner (I bought this refurbished), which bothered me but that's a whole other issue. They said the only option was to reinstall Vista since system restore wasn't working. (Found out later, yes, RA Media Services does have something to do with Remote Access. Thanks.)
After giving up on them, I found this helpful site to try to reset passwords. I tried it on the RA Media Services login. It looks risky, but I had nothing to lose at this point.
Offline NT Password & Registry Editor
Did not work for me, but I'm glad I know it's there in the future!
So I am screwed, right? I eventually decided to try the Vista reinstall disk and see if it had other options. Bingo. It has an option for System Restore that was able to restore me to a previous installation. No thanks to Dell support who specifically told me no. I was able to restore to the day before and voila, all is well. This took literally 4 hours or so, so I hope my experience can help others.