You can follow the steps in the following KB article and see if it resets the permissions properly.
How to reset security settings back to the defaults
http://support.microsoft.com/kb/313222
To find out exactly what directory/registry setting is getting access denied, you could run Process Monitor and then during the backup and look at the log file to see if you find Access Denied.
Process Monitor v1.26
http://technet.microsoft.com/en-us/sysinternals/bb896645.aspx