According to technet.microsoft website about IPv6 for Microsoft Windows:
IPv6 in Windows XP and Windows Server 2003 uses the “Automatic Tunneling Pseudo-Interface” for encapsulating IPv6 packets with an IPv4 header so that they can be sent across an IPv4 network. By default, IPv6 configures a link-local Intra-Site Automatic Tunnel Addressing Protocol (ISATAP) address on the Automatic Tunneling Pseudo-Interface. The link-local ISATAP address has the form fe80::200:5efe:w.x.y.x or fe80::5efe:w.x.y.x, in which w.x.y.x is an IPv4 address assigned to the computer.
You can remove this with this windows command:
