Microsoft Knowledge Base Article 825819 contains the following summary:
This article describes how to disable or remove Component Object Model (COM) Internet Services (CIS) (Windows NT 4.0 or Windows 2000) and RPC over HTTP
Proxy (Windows Server 2003) support.
RPC over HTTP version 1 (Windows NT 4.0, Windows 2000) and version 2 (Windows Server 2003) introduce support for a new remote procedure call (RPC) transport protocol that allows RPC to operate over Transmission Control Protocol (TCP) port 80. Version 2 also allows RPC to operate over TCP port 443. Operating over TCP ports 80 and 443 allows a client and a server to communicate in the presence of most proxy servers and firewalls. COM Internet Services allows DCOM to use RPC over HTTP (referred to as the Tunneling TCP/IP Protocol in the DCOM Configuration utility \[Dcomcnfg.exe\]) to communicate between DCOM clients and DCOM servers.
For additional information about COM Internet Services and RPC over HTTP, see the "References" section later in this article.