Cooling fans are mechanical devices that eventually wear out. Sometimes they fail catastrophically, but usually they just run slower and slower, eventually not moving enough air to keep critical components cool. Key fans in your system include the CPU fan, your graphics card's fan, and the power supply fan. The CPU fan is easiest to replace, and it's not hard to replace a graphics card fan. If a power supply fan dies, it's probably best to just get a new power supply.