Internet Information Server is not supplied with XP. It is only available on server installations, although apparently it can be installed on XP from the server installation disks. One caveat is that when it is installed on work station computers, it does not support multiple domains from a single IP address.
J.A. Coutts
Yes it is (with a disclaimer... don't stop reading here). For all versions of XP except Home (no, seriously, don't stop reading here). IIS is "Internet Information Services" and not "server" anymore, though and I have always considered "IIS" = "Microsoft Web Server" but that is not completely true. Anyway, Windows XP Pro x64 was based on Server 2003, and as couttsj stated, it may have hard limits of one web site and a particular number (IIRC 10) of simultaneous connections (again I am ignoring all the other IIS components and focusing on WWW). This was valid for the XP 32-bit versions, but I am unaware of XP Pro x64 having these limitations or if they exist.
Depending on where the OP is going with this and exactly what components are required, they may indeed need the Windows Server versions of IIS.