Unplug all of your USB devices, blow in the connectors and ports and put them back in. Then turn your keyboard upside down and gently hit the bottom of it in multiple places to make sure you don't have any stuck keys. Also make sure that you are not moving your mouse to the bottom right of the screen, because the very right of the taskbar (some call it the start bar) is a Show Desktop button. If you have any input devices other than USB devices, try blowing out their connectors as well.
I have personally seen many things that could cause this, I just tried to suggest the most relevant fixes that don't require major changes.
Also try downloading Malwarebytes Anti Malware free (don't start a trial) and install it, making sure you...