According to Alex Kochis, Senior Product Manager at Microsoft, erroneous test codes were copied on to the Windows activation server that checks the authenticity of XP Professional online. The server could then no longer differentiate between legal and pirated copies of Windows. Even genuine customers can see only a black desktop and the prompt to buy a licensed Windows copy at the earliest.
In the meantime, the defective codes were removed. Users with a black Windows desktop should visit.

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the website genuine and click on 'Validate Windows'. The authenticity check will then be re-conducted and Windows will be unlocked. If this does not help even if you have procured a licensed Windows copy, visit see tips for troubleshooting and get technical support for the Windows Genuine Advantage program. We recommend not using dubious WGA deletion tools available on the Internet, since, without the authenticity check, you cannot download many updates like Windows Media Player 11 for XP from the Microsoft site.