activate the windows firewall (it is neccessary) and make sure that you get a teredo address by checking if there is a “Teredo Tunneling” entry under network adapters in the device manager (you may have to turn on “show hidden devices”). you can also put in the following line into a command prompt to see if it works:
Netsh Int Teredo Show Stat
they should be two IP-addresses in the stats if it works.