Since this is my first post, I'm going to take a brief second to state that I work for a major PC manufacturer as a senior phone tech support agent.
Yesterday evening I had 3 phone calls that I assisted agents with on similar issues.
The most amazing of which was a customer who had somehow lost the permission to view his own pictures with Windows Photo Gallery.
I can't state that the following would work for everyone, but this, at least at the time, worked for the customer.
I enabled the hidden administrator account in Vista from a command prompt that was ran in elevated mode. (cmd.exe Right-click, "Run as Administrator")
net user administrator /active:yes
I then instructed the customer to log off of the account he was logged into and to log into the now accessable Administrator account, which was not password protected.
We reentered the eleveated command prompt on the Administrator account (Which to my understanding, elevated operations for certain functions is required when the User Account Control is active. Its part of its security.)
net user accountname /active:no
net user accountname /active:yes
Customer logged off the Admin account, back into his normal profile, and he had his access as the admin back. And for security purposes I had him disable the Administrator account again.
Again, I apologize if this doesn't work for you in the future, but for the customers I helped yesterday evening it worked.