Thank you for giving me a hint.

Quote Originally Posted by theMIROn View Post
Since you're able to ping ipv6 hosts from router itself your tunnel is up and working.
To get ipv6 cennectivity on your lan hosts (pc/notebooks) they have to receice ipv6 RA messages from router. It's used for neighbor discovery and socializtion, so, maybe your pc's firewall blocks them all.
And, I sure, you didn't forget about installing ipv6 transport proto in winxp, and to wait some time to let your local ipv6 address be autoconfigured.
Just check ipv6 ip and route table in winxp
When I connected to ipv6.google.com without router I checked firewall
settings (in my case BitDefender Total Security 2009) and added
all addresses (including IPv6) in BitDefender's "trusted zone".
And it works.

Now, when I connected via router I found some more addresses in
BitDefender's settings and added them all to the "trusted zone".
And it does not work.

After that I did some work around (still suspecting my firewall) and found
out BitDefender's "stealth mode" and turned it OFF.

And it works now.