By default, an IPv6 stack is not installed in Windows XP. In Windows 7, the network adapter already has "Internet Protocol Version 6", which Properties you can open and configure IPv6 address:

In Windows XP the network adapter has only "Internet Protocol" which is v4. To install IPv6, run this command in the command line:

netsh int ipv6 install

This will enable IPv6, and you can see it in network adapter properties:

However, the button to configure an IPv6 address is disabled... So it must be configured via command line:

netsh int ipv6 set address interface="LanGig" address=3ffe:1900:4545:3:200:f8ff:fe21:67cf

where "LanGig" is the name of interface you see in "ipconfig" output. Then, check that an IPv6 address is really configured:

netsh int ipv6 show address interface="LanGig"
Interface 6: LanGig
Unicast Address : 3ffe:1900:4545:3:200:f8ff:fe21:67cf
Scope : Global

or in classic way:

ipconfig
Windows IP Configuration
Ethernet adapter LanGig:
Connection-specific DNS Suffix . :
IP Address. . . . . . . . . . . . : 10.0.0.2
Subnet Mask . . . . . . . . . . . : 255.255.255.0
IP Address. . . . . . . . . . . . : 3ffe:1900:4545:3:200:f8ff:fe21:67cf
IP Address. . . . . . . . . . . . : fe80::211:d8ff:fe29:a5f6%6

As well, check if you can ping another IPv6 computer (Windows 7 in this case):

ping 3ffe:1900:4545:3:200:f8ff:fe21:67ce
Pinging 3ffe:1900:4545:3:200:f8ff:fe21:67ce with 32 bytes of data:
Reply from 3ffe:1900:4545:3:200:f8ff:fe21:67ce: time<1ms
Reply from 3ffe:1900:4545:3:200:f8ff:fe21:67ce: time<1ms
Reply from 3ffe:1900:4545:3:200:f8ff:fe21:67ce: time<1ms
Reply from 3ffe:1900:4545:3:200:f8ff:fe21:67ce: time<1ms
Ping statistics for 3ffe:1900:4545:3:200:f8ff:fe21:67ce:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 0ms, Maximum = 0ms, Average = 0ms

That Windows 7 machine has this IPv6 address configured:

ipconfig
Windows IP Configuration
Ethernet adapter LanGig:
IPv6 Address. . . . . . . . . . . : 3ffe:1900:4545:3:200:f8ff:fe21:67ce
Link-local IPv6 Address . . . . . : fe80::9d63:4580:a415:d3f8%11
IPv4 Address. . . . . . . . . . . : 10.0.0.1
Subnet Mask . . . . . . . . . . . : 255.255.255.0

and can ping the Windows XP IPv6 address:

ping 3ffe:1900:4545:3:200:f8ff:fe21:67cf
Pinging 3ffe:1900:4545:3:200:f8ff:fe21:67cf with 32 bytes of data:
Reply from 3ffe:1900:4545:3:200:f8ff:fe21:67cf: time<1ms
Reply from 3ffe:1900:4545:3:200:f8ff:fe21:67cf: time<1ms
Reply from 3ffe:1900:4545:3:200:f8ff:fe21:67cf: time<1ms
Reply from 3ffe:1900:4545:3:200:f8ff:fe21:67cf: time<1ms
Ping statistics for 3ffe:1900:4545:3:200:f8ff:fe21:67cf:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 0ms, Maximum = 0ms, Average = 0ms

so everything works with IPv6... Well, almost. Windows XP Task Manager does not show IPv6 traffic, but rest assured that's just a bug, and traffic is actually hitting the network.