Hey,
Go to task manager (CNTRL+ALT+Delete>>task manager) and then go to the "Processes" tab and click on "CPU" and see which process has the most cpu usage (ignore System Idle Process).
Report back with the name of the process that is using that much CPU usage. Do note that hafijur said your CPU is old, but if this has started to happen recently it could be an update, virus, or just a regular process.
Hope this helps!