Hi John ,
The error probably occurs because this account doesn not have permissions to access the Websites folder of your Sitefinity installation, or to access the database. Can you please check what type of permissions you have set for this account, both for folders and DB login? If permissions are set correctly and you still get the error, we would need more information about you machine setup. What is the version of IIS, how did you set up IIS authentication, do you use windows authentication for the database, or SQL server authentication?
Thank you for your cooperation in advance.
the Telerik team