Well, first of all you need a provider that supports IPv6. Only special providers actually do, unfortunately. If your provider does not, write him to complain, and get an IPv6 tunnel for example at
http://www.sixxs.net/ . Next: Windows XP does support IPv6, see for example
http://www.join.uni-muenster.de/Dokumente/Howtos/Howto_install_IPv6.php?lang=en#WindowsXP
Support is quite "poor" on XP, though.
Last but not least, our IPv6 DNS (pokerth.6dns.org) is currently down, so internet games through IPv6 are not possible at this time. This will be changed in the next release.