If the computer is a few years old, and a preventive maintenance hasn't been performed on it, then it's time for one.
Clean the inside of the computer. Remove the CPU Fan and Heatsink, remove the thermal paste from the Heatsink and CPU surface, and apply new thermal paste.
I recommend using a dust blower to remove the dust.
Be sure to unplug the PC from any power sources and external devices before performing the above.