By default IPv6 is not enabled on windows XP Pro and Windows Server 2003. However, its very easy to enable IPv6 with netsh (net shell) command line tool.
netsh interface ipv6 install
netsh interface ipv6 uninstall
To verify if ipv6 is enabled, run ipconfig /all and see if ipv6 address is returned.
Note: you can use netsh to enable/disable ipv4, just change ipv6 to ipv4 in above commands.