Why do my clients loose their MSN Messenger connection when the L2TP connection is opened?

In the properties of the VPN connections TCP-IP settings, there is an option to "Use default gateway on remote network" enabled by default. Enabling this option means that all internet traffic goes through the L2TP tunnel and out to the internet. The client logs on to messenger using one IP address, but after connecting to the L2TP tunnel, the client IP address changes (As seen by MSN Messenger server) and the connection is broken.

Go into properties for the connection, networking tab, select internet protocol (tcp/ip), hit properties, click advanced, and untick the box.