Microsoft Knowledge Base Article 817778 contains the following summary:

This article discusses the Advanced Networking Pack for Microsoft Windows XP Service Pack 1 (SP1). The Advanced Networking Pack is an add-on for Windows XP SP1 that includes an updated Internet Protocol version 6 (IPv6) networking stack, an IPv6 Internet Connection Firewall (ICF), and the Microsoft Peer-to-Peer (P2P) networking platform. The optional Windows Peer-to-Peer networking component provides a framework for peer-to-peer programs to communicate directly between computers over Network Address Translation (NAT) devices and over the Internet.

This article contains information about how to install the Advanced Networking Pack for Windows XP and the Windows XP Peer-to-Peer networking component, discusses IPv6 Teredo NAT traversal technology, lists some known issues, and describes how to troubleshoot these issues.