Your computer's central processing unit is effectively the machine's brain. It coordinates activity between your applications, your operating system, your hardware, and everything else on which your computer relies to function. If your CPU is stuck at 100 percent usage, you'll experience dramatic slowdowns with possible crashes and freezes. Reducing CPU use demands that you take careful stock of your running applications and the state of your computer hardware.
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Open your PC's Task Manager application by pressing "Ctrl-Shift-Esc." Task Manager can sometimes take a few seconds to appear. Alternatively, press "Ctrl-Alt-Delete" to open the Windows system menu, then click "Task Manager."
Click the "Processes" tab. Next, click "CPU" to sort the list of applications by CPU usage.
Read through the running processes to see which one is using the bulk of your CPU power. Depending on your PC setup, 100 percent CPU usage should not be occurring unless a program is updating or performing heavy calculations (Windows Updates, for example, or 3D animation software creating rendered images).
Right-click the offending application and select "End Task" to shut it down.