Internet Information Services (IIS) 5.1 is the web server supplied with Windows XP Pro. It is a cut down version of IIS6 and supersedes PWS (Personal Web Server) found in Windows 98/2000. The instructions below relate to installing IIS 5.1 on Windows XP Pro ONLY.
Note that IIS 5.1 is NOT included with Windows XP Home! For more information please see this Knowledge Base Article. Apparently you can get IIS 5.1 running on Windows XP Home - trying Googling for "running IIS on XP home edition". Alternately, you might want to consult this article.