My name is George M., and my old Windows XP had that problem. Somehow, it happened, and then i never got on to it again. When i started to try to fix it, nobody else had that sort of solution. But this is what i did. If nothing pops up, only you desktop wallpaper, then press Ctrl.+Alt+Del, and when you do that, you click on task manager. THen, you go to applications, and END ALL OF THE applications that are run by the USER, except the one with the task manager. THen, i pressed File, and then New Task, and then put in explorer.exe . Once that happens, it should pop up. If it dosent, restart the computer, and do the prosses all over again. Then it should work. But if it dosent, you computer is gone. On mine it works, But you will have to do that every time you shut down a computer. I always put mine on hibernate, so i don't have to do it all the time.
That is how i did it.