In the Wndows Genuine Advantage FAQ located at http://www.microsoft.com/genuine/do...FamilyID=321cd7a2-6a57-4c57-a8bd-dbf62eda9671
in response to one of the questions it mentions that validation is
recommended (but not required) for Windows XP but then it goes on to say
that if validation should be required in the future for updates that it
would be extended to include Windows 2000 (client not server).