IPv6 with XP is not fully supported by Microsoft. Because of that not many people have been very persistent on finding workarounds which is why I was working on getting it as close as reasonable. Its certainly fine for dual-stack networks but you could have issues with IPv6-only networks for some things.
First, install all critical Windows updates after upgrading to the latest Service Pack (SP3). Otherwise don't even bother with IPv6 on XP. Seriously. There's going to be too many issues until then.
Install base IPv6 support:
From GUI (Start -> Control Panel -> Network Connections -> right-click a connection -> Properties -> [Install...] -> Protocol -> Microsoft TCP/IP version 6)
From command line (netsh int ipv6 install - note: is preferred to not use ipv6 commands without netsh)
Enable all ICMP traffic for both v4 & v6. Control Panel -> Windows Firewall -> Advanced -> ICMP -> Settings... Have everything checked -> OK -> OK
- Essential for IPv6
- Blocking (even if just for IPv4) is inappropriate since causes more problems (broken route diagnosis & PMTUD) than it helps (securing from DoS attacks). Have your main firewall rate limit ICMP instead.
DNS lookups do not work over IPv6 only but for dual-stack networks this doesn't matter since DNS servers will return AAAA (IPv6) records just fine over IPv4.
Check if DHCPv6 is being used with your network for assigning IPv6 addresses. If so add DHCPv6 support via a third-party client (Dibbler or DHCP Turbo/DHCP Broadband).
Update default policy table to prefer IPv6 better or if you won't do that and you have multiple IPv6 connections then your system might use the wrong one so instead you should:
Disable Teredo and/or other transitional technologies with noticeable packet overhead (see: http://www.sixxs.net/faq/connectivity/?faq=comparison) unless needed for NAT traversal & certain of no better options
Note of Teredo with XP issue: http://yorickdowne.wordpress.com/2008/01/26/ipv6-at-home-part-1-overview-teredo/
Type this: netsh int ipv6 set teredo disabled
Disable 6to4 if you can use anything else for IPv6 (e.g. managed tunnel broker if not native) and your ISP doesn't have their own relay router (which without it the outbound route is random)
Disable ISATAP if not provided on your LAN (is useful for connecting to another IPv6 LAN with IPv4 Internet in-between)
Manually enable for 1.x .NET Frameworks (very optional & link upcoming)
Restart then test
Known IPsec issues:
IPsec for IPv6 in Windows XP and Windows Server 2003 is not centrally configurable and deployable
http://technet.microsoft.com/en-us/library/cc736336%28v=WS.10%29.aspx#BKMK_IPsec
IPv6 PPPoE - probably need to use software (cFos IPv6 Link or alternatives) if you have dial-up or have your router/modem handle it for other types
Internet Connection Sharing (ICS) doesn't share an IPv6 connection properly. For some reason it causes timeouts for IPv6-enabled domains on Windows clients from what I've found.
Need some more testing on IPv6 support with various other Windows system protocols like RPC, NTP, RDP, and other Windows services
IPv6 Support in Microsoft Products and Services - applies for newer versions so if not listed as supported here then definitely wouldn't be in XP-era ones
http://msdn.microsoft.com/en-us/library/gg134033.aspx - list
Most of this likely applies for Windows Server 2003 too.