An open case can make things a lot cooler — if you put a full-size box fan right next to it. Otherwise, your friend's system is probably sucking in lots of dust, bugs and pet hair in exchange for a marginal or nonexistent cooling increase.

In a properly designed PC case, the fans create a ducted airflow that enters in the front, passes over the CPU, power supply and other heat-producing hardware (components that often have their own heat sinks and fans), and exits out the back. By controlling fan speeds and where air leaves and enters the case, the engineers can guarantee that you're pulling in fresh, cool air and ducting out hot air — rather than fueling a vortex that spins the same hot air around the quickly heating CPU.

In other words, a closed case actually helps direct the air to where it should go. So tell your friend to close his case and make sure that he doesn't have any obstructions that would impede the flow of air from the front to the back, such as extraneous cables.

Don't believe me? Many free software programs, such as SpeedFan, allow you to poll your PC's built-in temperature sensors to do your own testing.