I just applied a windows update and thought nothing of it. Suddenly nobody can login without this error other than system administrators. Best solution I've seen so far for this problem is here:
microsoft.public.windows.vista.general | Google Groups
Don't know if it works or not yet. Seems you have to make everyone an administrator, login as them and then after you've done that you might be able to turn off those rights.
I've never had a problem like that for any other operating system I've used (and I've used quite a number of them). As the saying goes, better anything than vista!
Mike