Standard PCs are only equipped with cheap coolers
The standard coolers, which you can find in most PCs, are not suitable at all for overclocking. Who plans to tune up his system by overclocking has also to consider that the CPU gets a powerful cooler. Otherwise the CPU will heat up too much by overclocking and a cheap cooler cannot handle these amounts of heat.
You can easily see the difference between quality models and no-name products as you can find them in discount PCs. The heatsink of every cheap cooler is usually made of cast steel. It works with a fan of simple design which has to work without any special bearing mechanism. The parts are made of plastic often show burrs and a good contact between cooler and CPU cannot always be guaranteed.
Battle against thermal death: The optimal CPU cooler
How can you characterize the optimal CPU cooler? For this you have to know that there are two thermal interfaces. One is the junction between the die of your CPU and heat spreader, the other is the thermal resistance between surface of the cooler and the ambient air.
Let's have a look at the heat spreader that touches the die of your processor. You have to make sure that this junction has a good thermal conductivity. Any gaps impair the cooling effect, no matter how good the cooler is designed.
The major role is played by the spring bracket that you attach to the socket in order to press the heat sink onto the die. Sometimes, micro-gaps can't be avoided. Perfect smoothness can never be achieved because any material has certain ruggedness on the surface. Furthermore, the pressure of the bracket is unequally distributed.
You have to apply cooling paste to seal off any micro gaps. An alternative is also a thermal pad, which is basically a piece of adhesive tape with high thermal conductivity stuck to the heat spreader.
Now the distributed heat has to get absorbed by the ambient air. Two criteria are important - the surface area and the air flow. If you believe it or not, the material only plays an inferior role here. For example a copper heatsink can be worse than an aluminum one if the surface area of the copper cooler is smaller. Our tests prove it as you can see in the results.