ClickOnce installed apps are hidden deep under \Document and Settings\UserName\ \Local Settings\Application Data\CompanyName\ApplicationName_StrongName\ApplicationVersion\...

 

If you want to remove a ClickOnce application, you should use Add/Remove Programs in Control Panel.  However, I’ve seen cases where that method just doesn’t work, and you might get “Application cannot be started” message…  In this case, you can run Mage.exe –cc command to clear the application cache on that machine.  The mage.exe utility comes with .NET Framework SDK…  Beware:  it will clear the entire application cache, not only for the failing application!