.NET Framework

/// Since this assembly is being run from the install directory, it will return the full
/// path to this assembly. You can simple get the directory path using a FileInfo object
/// to know where it's been installed.
System.Reflection.Assembly asm = System.Reflection.Assembly.GetExecutingAssembly();
string baseAppDir = (new System.IO.FileInfo( asm.Location )).DirectoryName;
 
/// We're certain this exists because our setup program created it.
RegistryKey softwareKey = Microsoft.Win32.Registry.LocalMachine
               .OpenSubKey( "Software", true )
               .OpenSubKey( "Company Name", true )
               .OpenSubKey( "Application Name", true );
 
softwareKey.SetValue( "InstallLocation", configFilepath );