Boxed processors come complete with a heatsink/fan (HSF) assembly, together with a pre-applied thermal interface material (TIM). This is usually satisfactory for everyday use, and also for minor overclocking. Serious overclocking will require a better heatsink, in which case you will need to apply TIM, but do not use too much as that can be just as bad as not using any at all. Should you change your processor but decide to keep your old HSF, you will need to remove the old TIM and replace it with fresh. This applies equally to a HSF with a pre-applied pad as it does to one where you have previously applied TIM. In actual fact, it is generally recommended not to apply TIM to the heatsink, but to apply a very thin layer to the top of the processor itself.