A few months ago I already had problems with the Microsoft authentication wga in my WinXP Professional. At that time I was able to solve the problem with the help of my WinXP salesperson.
Now this WGA message "... This copy of Windows has not passed the Windows authentication" annoys me together with a changed (black) desktop again. I can't think of any reason for that.
Since the WGA test should, for example, take offense at a replaced motherboard, could it be that the Windows authenticity test assumes that it is a new computer and thus a second, unauthorized installation, even if the mainboard battery has become weak?