I guess that you, your collegues have access to MS TechNet Forums - there you can find this theme discussed under “Windows Vista Setup-” Forum.

As far as I can figure out - it is the Vista one that goes… and there has been the suggestion that if one switches off the system-restore on the xp - the vista one can be preserved (and xp is gone of course…) All this does not help when the incident has already occured with no access to a sys-rest-point.