It appears that under vista, this points to a registry key under HKEY_USERS that is NOT the same for all user accounts, i.e. if your app writes to something below this key, it will find different data at this location per user in Vista. Presumably this is because writing to HKLM needs elevated privileges in Vista, and this was a less-intrusive workaround than spawning the User Accounts Control notice every time an existing app tried to write here.