If you get the following error:
The recommended solution is to clear the ClickOnce application cache under the ClikOnce cache folder. If the OS is Windows 7 or 8, the folder should be:
The user should delete everything that can be deleted under the 2.0 directory.
If you don't see the 'appdata' directory you need to make hidden directories visible. Select the 'Hidden items' checkbox in Windows Explorer under the View menu.