Microsoft Knowledge Base Article 813878 contains the following summary:
Internet Protocol security (IPSec) filtering rules can be used to help protect Windows 2000-based, Windows XP-based, and Windows Server 2003-based computers from network-based attacks from threats such as viruses and worms. This article describes how to filter a particular protocol and port combination for both inbound and outbound network traffic. It includes steps to whether there are any IPSec policies currently assigned to a Windows 2000-based, Windows XP-based, or Windows Server 2003-based computer, steps to create and assign a new IPSec policy, and steps to unassign and delete an IPSec policy.