I ran into this problem myself for no apparent reason…
Well now there’s a fix! Thanks to a regular Vista user on Microsoft’s TechNet forums, a fix has been found!
For Business and Ultimate version users:
>>Right click “Computer”
>>Click “Manage”
>>Under the “System Tools” section, Double click “Local Users and Groups”
>>Click “Groups”
>>Right click “Administrators”
>>Click “Add to group…”
>>Click “Add”
>>Click “Advanced”
>>Click [...]