Just found this old thread which may apply to a new problem I am facing - can't seem to make IPv6 work in a new virtual machine w/ SP Pro SP3 (a Virtual PC downloaded from and preconfigured by Microsoft). The VM indeed had the Update in subj applied, which I simply can't uninstall lacking the Uninst folder.
That VM is a replacement for a previous version of that Microsoft VM (lost in a disk crash), IPv6 worked without problem in the previous VM. All other things being unchanged I /conjecture/ the problem could come from the offending XP Update, which possibly the lost VM did not have applied.
Willing to try the "hosts" file approach : here is a (maybe dumb) question, in the domain name mentioned up-thread : " isatap.yourdomain.com",
is "yourdomain.com" a (magic) predefined litteral, to be entered as such in the Hosts, or is it rather a placeholder for "my(?)domain" or else again is it related to the tunnel broker ?
My home network is /not/ a "domain" (in the sense of MS), and I don't run a DNS server on the LAN, if this matters. As such I wonder what "mydomain" is if it isn't 'mydomain'

The (virtual) XP machine is supposed to get its v6 configuration through RA. The IPv6 gateway is a (physical) computer running Win 2k or sometimes Linux. Other than this new VM, I have IPv6 related problem on the (small) LAN here.
[Edit]
1. Problem solved by reapplying the solution that... /I/ provided way-back
http://www.tunnelbroker.net/forums/index.php?topic=573.msg2477and had the time to forget since...
2. Although the ISATAP on XP was /not/ the problem, finally (not used in this config indeed), I'm still interested in an answer to the clarification request I posted above, regarding the DNS or host.