>>>How do you determine in C# if an application is installed?
>>
>>The answer to that will depend on the application in question and, if it is installed, how the installation was carried out.........
>
>Skype. Windows installer.

Works for me but no quarantees :-}

public static bool IsSkypeInstalled()
        {
            using (RegistryKey rk = Registry.CurrentUser.OpenSubKey("Software\\Skype"))
            {
                return rk==null? false :
                (from string x in rk.GetSubKeyNames() where x == "Phone" select x).FirstOrDefault()!= null;
            }
        }