Today an internal build of Windows Live Messenger 8.5 was leaked by Messenger Addictos, a site dedicated to Windows Live Messenger and related products.

The new build includes a completely new installer system (though it appears it might need some tweaking). Messenger itself also got a complete overhaul, featuring a new UI that combines the best of Windows Live and Vista. The new build also includes several minor new features, some of which are still unknown. Unfortunately support for Plug-Ins (or "Add-Ins") has been dropped.

The leaked build is only available in Spanish at the moment, but if you managed to download it somewhere, you can copy the old language file ("msglang.dll") of Windows Live Messenger 8.1.0178 over the new one ("msglang.8.5.1235.0517.dll"). Note that the installation directory has also been changed to "C:\Program Files\Windows Live\Messenger\" making the transition from MSN Messenger to Windows Live Messenger complete.