What to do if High Windows CPU Usage? (opens in new tab)

My Windows computer is experiencing very high CPU usage that stays at 90 to 100 percent constantly even when I have no applications open. The system becomes sluggish and unresponsive and the fans run loudly all the time. I have checked Task Manager but the processes listed don't seem to add up to the total usage percentage being reported. This happens after startup and continues for hours without stopping. I have already scanned for viruses and updated my drivers but nothing has fixed the pro...