Most pirate versions of Windows XP are copied from a small number of legal discs and Microsoft has worked out the serial numbers... of the 20 most copied. Service Pack 2 adds new security features to Windows XP. It will be available from July as an 80Mb download or on CD from leading retailers.
The same tactic was used to stop pirate copies from installing SP1, although pirates then launched a tool to get round the anti-piracy measures.
Users who have versions of XP bearing any of those serial numbers will be blocked from installing Service Pack 2.