Click Once fails!

If Click Once fails, it helps in the most cases to clear the click once cache to solve the issue:
rundll32 %windir%\system32\dfshim.dll CleanOnlineAppCache

If this does not help, please try to delete the cache manually:
Windows Vista and higher: %userprofile%\AppData\Local\Apps\2.0\
Windows XP: %userprofile%\Local Settings\Apps\2.0\

Just delete the content of the 2.0 directory.

Please keep in mind that roaming profiles are not supported:

The Internet Explorer can run the click once technology by default.
To use Click Once in Firefox, you need to install the Microsoft .NET Framework Assistant Addon first.

If Click Once is not deployed directly in the browser and a download starts, the Firefox addon is not installed correctly or it is not active. You can check it over about:addons