Microsoft Knowledge Base Article 323381 contains the following summary:

This step-by-step article describes how to configure a computer that is running Windows Server 2003 to allow remote users to establish an encrypted channel to a corporate network. Users can connect to a remote access server through a dial-up connection or a virtual private network (VPN) connection.

A dial-up connection requires both the server and the client computer to have a correctly configured modem. The client and the server connect over analog public telephone networks. To enhance the security of a dial-up connection, use data encryption, Windows logon and domain security, remote access policies and callback security.

A VPN connection is made over a public network, for example the Internet, and uses Point-to-Point Tunneling Protocol (PPTP), logon and domain security, and remote access policies to help secure the transfer of data.

The scenarios that are described in this article assume the following configurations:

For dial-up connection capability, the modems are configured on the server.
For VPN capability, the server has two network adapters, with one of them connected directly to the Internet.
For VPN capability, PPTP is used for the VPN tunnel.
No routing protocols, such as Routing Information Protocol (RIP) or Open Shortest Path First (OSPF), are configured.