This is an L2TP error. You will most likely get this if you attempt an L2TP connection from Windows XP. On Windows XP you will need to download and run this registry file and restart your computer. Please note that a restart is mandatory. You may also choose to use a different VPN connection method like PPTP.
Once running the registry file and rebooting, it is not possible to get error 789. You may receive a different error code, but not error 789. If you continue to receive Windows 789 on XP with L2TP, you made a mistake in the solution procedure above.
It is possible to get error 789 on Vista and Windows 7, using a misconfigured VPN connection, created with the built in connection wizard. On Vista and Windows 7 you must always use the connection tool or pbk file provided by the VPN company if you want to use L2TP.
PPTP will never give this error on any Windows version.