If you receive the following exception even though your VSTO ClickOnce application is already uninstalled:
"Unable to install this application because an application with the same identity is already installed. To install this application, either modify the manifest version for this application or uninstall the preexisting application."
Then you may have issues with your ClickOnce application Cache. You have 2 options for clearing it:
Running the following at a Visual Studio Command Prompt:
Running the following at a command line on any machine with the problem:
rundll32 dfshim CleanOnlineAppCache
See http://vijayvepa.wordpress.com/2011/01/11/unable-to-install-this-application-because-an-application-with-the-same-identity-is-already-installed-to-install-this-application-either-modify-the-manifest-version-for-this-application-or-uninstall/ for more details.