If you receive the following error:

Exception Source: mscorlib
Exception Type: System.
UnauthorizedAccessException
Exception Message: Access to the path is denied.
Exception Target Site: WinIOError

This means Invoice Expert doesn't have permission to access a folder. One solution is to run Invoice Expert as an administrator. You can do this by right clicking on Invoice Expert before you run it and selecting "run as administrator". On Windows 8, you need to right click on the desktop shortcut to Invoice Expert (the start menu shortcut won't have the option to run as administrator).