Sometimes Windows messes up when it logs on to your user account – the "Preparing your desktop" message is a giveaway it's setting up a temporary user profile.

Try pressing [Ctrl] + [Alt] + [Del] at this point – if you're lucky, you can log off and try to log on again. This error can occur if you try to log on too quickly after the Windows Welcome screen; wait ten seconds before entering your password and hitting [Enter].