Open task manager (ctrl + chift + esc)
Click the processes tab
Sort by CPU (click the CPU tab once or twice so the highest cpu-using programs are listed up high)
Look at what is using a high amount of cpu, google it.
Like right now, firefox is using a high amount of cpu, so I'll google "firefox.exe"

Most likely you either have a virus, or your antivirus program is running and scanning your computer.

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