Several circumstances can make your PC overheat, many of which compound one another to make the problem worse. You can mitigate computer overheating, primarily by having good maintenance habits. Before cracking open your casing to install a new high-end cooling system, try a few heat-dispelling tricks.
Your computer's location plays a large part in its ability to self-cool. Poorly ventilated rooms or ones that receive minimal climate control are unfriendly computer environments, especially during the summer. If the computer is in direct sunlight, this can also significantly increase its temperature. Keep your computer away from windows and other light sources and in a temperature-controlled environment.
Poor ventilation can also be a significant cause of overheating. Most computer cases have vents that allow the passage of air into and out of the casing. If these vents are covered due to the location of the computer, dust accumulation or other reasons, this critical airflow is stopped and warm air will build up inside the casing. Keep the vents clear to allow the passage of air to and from the casing to minimize overheating.
Dust buildup on both the vents and the internal components can also contribute to overheating. While dust buildup on the vents blocks airflow, dust on the internal components of the computer provides insulation and traps heat. This can be particularly problematic on components that regularly generate large amounts of heat anyway, such as the CPU and the power supply. Periodically clean your computer's interior using a can of compressed air, taking care not to spray any of the components too directly or too harshly.
Several components of your computer generate heat during the course of normal operations. Chief among these are the CPU and the power supply, which both generate substantial heat. Under most circumstances, their heat generation is mitigated by the computer's cooling system and proper ventilation.
Malfunctioning or Insufficient Cooling
All computers come equipped with some manner of interior cooling system. In most cases, this system is comprised of a small fan, which increases air circulation throughout the casing and may or may not be directly pointed at heat-producing components to lower their temperature. If your computer's cooling system is not functioning properly or is insufficient for your cooling needs, it is not able to properly mitigate the heat generated by your computer and likely needs service or replacement.