You should see a version of the image above which is a list of the currently running processes on your machine. Clicking any of the column headers within the window, where you see Image Name, User Name etc, will sort them by name, or number. Clicking on the CPU header will bring those processes who are monopolizing the processor to the top. In most situations you want to see a process called System Idle Process at the top. This is a Windows process that runs dummy instructions through the processor to allow it to use power saving features like Speedstep.
If you find your processor running a lot at 100%, Task Manager will give you an idea of what is going on right at that moment. If you want to take a longer view and get more of an idea of what has been going on, rather than what is going on, you need to use Perfmon.
Perfmon is a program built into Windows that can give you an array of performance information about your machine. Actually called Performance Monitor, it can be invoked by typing perfmon into the search bar or command prompt.
You should then be presented with a larger window with a red line graph showing your processor utilization. Highlighting the Performance Monitor tab on the left side will bring up the application in full.

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