JSI Tip 4099. How do I join or access an internal domain from an external client using ISA Server and VPN?

Microsoft Knowledge Base Article 303503 contains the following summary:

This article describes how to join or access an internal domain from an external client or server by using a virtual private network (VPN). This article describes how to set up the VPN (PPTP) with Internet Security and Acceleration (ISA) Server acting as a firewall (integrated mode) when the external computer is outside the local address table (LAT), while the domain is inside the LAT.