Back on June 21 I posted about IPv6 teredo interface not working on Windows 7 and how to correct this using commmand prompt netsh commands.
Just in case anyone is having the same problem on Windows XP, here is the solution for it. The problem is the same, the commands are slightly different.
Command for showing the teredo interface state:
netsh interface ipv6 show teredo
If it says the state is offline with error of none, try this:
netsh interface ipv6 set teredo enterpriseclient
That should do it. The IPv6 network components were incorrectly thinking your computer was in an enterprise situation.