If upon startup your Windows wallpaper is cut reduced to a line at the top of screen
You may find yourself in this strange situation. You defined a slideshow of your pictures as desktop background, but when Windows starts, instead of the last open background you see only part of that picture at the top of your monitor. This ‘broken’ picture remains until the slideshow replaces it with the next picture of your slideshow.
To solve your problem you do not need to make registry changes or anything risky. Just open the personalization options and select a single picture as your desktop background. This will disable the slideshow.
Restart Windows and you will see that this background picture appears as it should. Then enter again personalization options and select again all your slideshow background pictures.
Restart Windows and enjoy your background slideshow without the broken picture problem.