I'm 99.9% sure I had the firewall on (I don't think I would have had the firewall off on an XP computer directly connected to the net) I think I only had to allow incoming ICMP so I could create the tunnel.
I didn't read back through the thread, but did you try a brand new XP install with your setup?
I used a fresh install from a CD slipstreamed with SP2, and updated from MS Update.
A brand new XP (SP zero) would not help diagnose the question, since it wouldn't have the new windows firewall - nor even IPv6 installed by default.
I don't think the problem is to do with something special in this install. But reading again your previous messages,
just connected directly to my internet router.
I wonder : were you NATting (IPv4) or not so ? IOW, in your experiment did that XP computer get the public IP from the ISP or did it get a private IPv4 address ? Notice how in my setting here, the XP comp gets a private 10.x.x.x addie from the Speedtouch ADSL "modem/router". This might be the crucial point...